Why does the Torah write "Kedoshim Yihyu" and not "Kedoshim Heim"?
Rashi: To teach us that the Kohanim must be forced to sanctify themselves in the above regards even if he does not want to.
Ramban: Because 'Kedushah' is synonymous with 'P'rishus', 1 and the Torah is therefore instructing the Kohanim to abstain from things that are permitted to a Yisrael - such as Tum'as Mes and marrying the women that the Torah lists.
To which Mitzvos does "Kedoshim Yihyu" apply?
Moshav Zekenim (8): It applies to shaving the corners of the beard (in the previous verse), like is clear from Nazir 1 but not to Zonah in the coming verse.
57b. Shi'urei R. Shmuel (Sanhedrin 18b) says that it applies also to Karachah in the same verse (and surely the same applies to Seritah. Yevamos 20a says that there is an Aseh of a Besulah widow to a Kohen Gadol ("Ishah bi'Vsuleha Yikach") implies that there is no Aseh of "Kedoshim Yihyu." However, the Rashba (20a) says that the Aseh "Kedoshim Yihyu" applies!