Is whose possessive form of who?

Yes, the pronoun 'whose' is the possessive form of 'who'.

The pronouns 'who' and 'whose' are both interrogative pronouns and relative pronoun.

Example as interrogative pronoun:

Who parked in our driveway?

Whose car is in our driveway?

Example as relative pronoun:

The one who parked in our driveway is the painter.

The one whose car is in the driveway is the painter.