When do you use apostrophe with possessive pronouns?
The only possessive pronoun that uses an apostrophe is one's, as
in "A room of one's own." All other possessive pronouns -- his,
hers, yours, ours, theirs, its, my, mine and whose -- do not have
apostrophes.
It's is the contraction of "it is." Who's is the contraction of
"who is."