1)
Why does it mention Chasid, Gevar and (in verse 27) Navar?
1.
Rashi, here and Shmuel II, 22:26 based on Targum Yonasan there: These correspond to the three Avos. 1 Hashem paid the reward for their virtue to their descendants, Midah k'Neged Midah.
2.
Radak (26-27, Shmuel II, 22:26-27): With a Chasid, You are a Chasid, and do good to him more than he deserves. Gevar is the Targum of "Ish" (Bereishis 2:24. He is pure, but not a Chasid - PF). You are pure with him.
3.
Malbim: With a Chasid, You do Chesed. If one is pure or Navar 2 , so You conduct with him, according to his preparation (mindset), for good or for evil.
