Is sam's a possessive pronoun

No, the Word Sam's (a proper noun requires a capital S) is a possessive noun.

A pronoun is a Word that takes the place of a noun in a sentence.

A possessive pronoun is a Word that takes the place of a noun that belongs to someone or something in a sentence.

The possessive pronouns are: mine, yours, his, hers, its, ours, theirs.

EXAMPLES

possessive noun: Sam's house is on the corner.

possessive pronoun: The house on the corner is his.