Gemara

20a (Mishnah): A widow to a Kohen Gadol (does Chalitzah, not Yibum).

Question: This is even if she was widowed from Kidushin, and only a Lav forbids her. The Aseh of Yibum should override it!

Answer #1 (Rav Gidal): "Yevimto will ascend to the gate" is extra, to teach that there are Yevamos who do Chalitzah but not Yibum, i.e. Chayavei Lavin.

20b - Question (Rava - Beraisa): If one did Yibum or Chalitzah with Isurei Mitzvah (Shniyos) or Isurei Kedushah (Chayavei Lavin), the Tzarah is exempted.

If mid'Oraisa Chayavei Lavin do Chalitzah but not Yibum, why does Yibum exempt the Tzarah?

Answer (Rava): The Beraisa taught separate laws together. Yibum of Isurei Mitzvah or Chalitzah of Isurei Kedushah exempts the Tzarah.

Objection (Rava - Beraisa): A Petzu'a Daka, Krus Shofchah, Seris Adam or (very) old man does Chalitzah or Yibum. If he gave to his Yevamah a Ma'amar or Get or did Chalitzah, it takes effect. If he did Yibum, he acquires her, but they may not remain married - "Lo Yavo Fatzu'a Daka...(bi'Khal Hash-m)".

If mid'Oraisa, Chayavei Lavin do Chalitzah but not Yibum, why does he acquire her through Yibum?

Answer #2 (Rava): We forbid Yibum mid'Rabanan, lest he have a second Bi'ah with her (which is forbidden).

Support (Beraisa): If they did Yibum he acquires her with the first Bi'ah. A second Bi'ah is forbidden.

Retraction (and Answer #3 - Rava): (Yibum is forbidden mid'Oraisa.) Reish Lakish taught that an Aseh overrides a Lav only if one cannot fulfill both. Here, both can be fulfilled through Chalitzah!

Refutation (of Answer #3 - Beraisa): If they did Yibum, he acquires her.

(R. Yochanan or R. Elazar): If a Kohen Gadol did Yibum with a widow, the Tzarah is exempted.

(The other of R. Yochanan and R. Elazar): She is not exempted.

If she was widowed from Nisu'in all agree that she does not exempt the Tzarah, for an Aseh does not override an Aseh and a Lav. They argue about a widow from Kidushin.

One opinion says that she exempts the Tzarah, because an Aseh overrides a Lav. The other opinion says that she doesn't exempt the Tzarah. The Aseh doesn't override the Lav, since Chalitzah could be done.

Refutation (of the latter opinion - Beraisa): If they did Yibum, he acquires her.

Suggestion: This refutes Reish Lakish (who says that when it is possible to fulfill both, the Aseh doesn't override the Lav)!

Rejection: No. Chalitzah is not considered a fulfillment of Yibum.

Rishonim

Rashi (20b DH O Meyabmim): The Beraisa says that a Petzu'a Daka does Chalitzah or Yibum. Yibum is Asur, but if done it acquires.

Hagahos ha'Gra [1]: The text should say 'he does Chalitzah but not Yibum.'

Rif and Rosh (2:1): Isurei Mitzvah or Kedushah (Shniyos or Chayavei Lavin) do Chalitzah but not Yibum. This includes a widow to a Kohen Gadol, whether she was widowed from Kidushin or Nisu'in.

Nimukei Yosef (DH Masnisin): Tzaros of Chayavei Lavin may do Yibum; only Ervah exempts. The Yerushalmi explicitly says so.

Gra (EH 174:2): The Tosefta (Sof 1:1) permits Tzaros to do even Yibum.

Note: Our text says 'they do Chalitzah but not Yibum'.

Pnei Moshe (Yerushalmi 11b DH Choletzes): The Bavli itself (20a) permits. (It says 'Regarding Isurim of Ervah the Tzarah is forbidden. In other cases, the Tzarah is permitted.')

Rambam (Hilchos Yibum 6:10): If the Yevamah was forbidden to the Yavam by a Lav or an Aseh or she was a Shniyah, she does Chalitzah, not Yibum. Because Kidushin takes effect on them, they are Zekukos to Yibum. According to letter of the law, Yibum is permitted, for Aseh Docheh Lo Sa'aseh. Chachamim decreed to forbid Yibum of Chayavei Lavin and Shniyos, lest he have Bi'ah with her a second time. The Mitzvah is only the first Bi'ah. Therefore, if he transgressed and did Yibum with a Yevamah forbidden by a Lav or an Aseh, and certainly if she is a Shniyah, he acquired her totally and divorces her with a Get. She and all the Tzaros are exempt from Yibum and may marry others.

Magid Mishneh: Rav Gidal's Drashah (there is a Yevamah who does Chalitzah but not Yibum) applies to a widow from Nisu'in to a Kohen Gadol.

Question (Lechem Mishneh): Mid'Rabanan, a forbidden Yibum does not exempt the Tzarah. Here also Yibum is forbidden mid'Rabanan!

Answer (Lechem Mishneh): We cannot compare decrees to each other. Chachamim decided to be stringent about Yibum after a Get was given. However, the Rambam (7:10) says that Chalitzah of Chayavei Lavin does not exempt the Tzarah. This is difficult, for we cannot learn from an improper Chalitzah (after a Get or Ma'amar was given)!

Rambam (6:11): If a Kohen Gadol did Yibum with a widow from Nisu'in, the Tzarah is not exempt. This is because an Aseh is not Docheh a Lav and an Aseh, so he did not totally acquire her mid'Oraisa. Therefore, the Tzarah needs Chalitzah.

Rambam (7:9,10): An improper Chalitzah (Yibum was forbidden) does not exempt the Tzarah. It seems that if a Yevamah was forbidden to the Yavam by a Lav or an Aseh and he did Chalitzah to her, he did not exempt her Tzarah. If he did Chalitzah to her Tzarah, he exempted her.

Magid Mishneh: When two sisters fall to Yibum, Chalitzah of one does not exempt the other, even though Achos Zekukaso is forbidden only mid'Rabanan. The same applies to Chalitzah of a Shniyah or woman forbidden by a Lav or an Aseh. However, BaHaG says that her Chalitzah exempts. Some explain that Chalitzah does not exempt the Tzarah only if the Isur Yibum is due to Zikah or a Get, Ma'amar or Pasul Chalitzah. This is a difficult distinction, but a Tosefta (6:5) supports it. The Rambam holds that since the Tosefta opposes Shmuel (27a) but we did not challenge Shmuel from it. A Beraisa (20b) says that Chalitzah of Chayavei Lavin exempts the Tzarah. We could say that this is when also the Tzarah is Chayavei Lavin. However, the Rambam says that even if this case the Tzarah is not exempt.

Bach (EH 170 DH v'Chosav`ha'Rambam): According to the Rambam, the Beraisa teaches that mid'Oraisa the Tzarah is exempted through Yibum or Chalitzah. It does not discuss the need to repeat Chalitzah mid'Rabanan.

Beis Shmu'el (13): How can we say that it is mid'Oraisa? The Beraisa also discusses Shniyos! Perhaps it gives the mid'Oraisa law for Chayavei Lavin, and the mid'Rabanan law for Shniyos.

Gra (45): We can say that the Beraisa holds that Ein Zikah, therefore Chalitzah exempts the Tzarah (Rashba 12a DH Taritz).

Poskim

Rema (EH 170:5): The need to repeat a Pasul Chalitzah is when there was a full Zikah and it became weaker. There is no need to repeat if the Chalitzah was Pasul from the beginning.

Source (Beis Shmu'el 4): Darchei Moshe (1) brings this from the Or Zaru'a. Also Tosfos (26b v'Chalitzah) says so.

Shulchan Aruch (18): If two Yevamos fell from one man, one a Shniyah to the Yavam and her Tzarah is permitted, Yibum may be done only to the Tzarah. Chalitzah may be done (even) to the Shniyah. Some say that Chalitzah of the Shniyah does not exempt the Tzarah, but Chalitzah of the Tzarah permits the Shniyah.

Beis Shmu'el (13): It is not clear why here the Shulchan Aruch considers the Rambam's opinion to be secondary (some say that...), but below (174:1) it brings only the Rambam's opinion. Also, why did the Rema rule like Tosfos above, but did not comment here or below?

Shulchan Aruch (174:1): If a Yevamah (Leah) was forbidden to the Yavam by a Lav or an Aseh or Shniyos mid'Rabanan, she does Chalitzah, not Yibum. Her Tzarah (Chanah) may do Chalitzah or Yibum.

Gra (1): Also when she is forbidden by an Aseh, the Isur Yibum is a decree lest he have a second Bi'ah.

Shulchan Aruch (ibid.): Chalitzah of Leah does not exempt Chanah, but Chalitzah of Chanah exempts Leah.

Gra (3): This is like the Rambam. The Poskim hold that in either case the Tzarah is exempt (170:18, Rema 170:5).

Shulchan Aruch (ibid.): If the Yavam transgressed and had Bi'ah with Leah, he acquired her and the Tzarah is exempt. He may divorce Leah with a Get.