pronoun vs function word

pronoun and function word both are nouns.

Word	Noun	Adjective	Verb	Adverb
pronoun	Yes	No	No	No
function word	Yes	No	No	No

As nouns, function word is a hypernym of pronoun; that is, function word is a word with a broader meaning than pronoun:

pronoun: a function word that is used in place of a noun or noun phrase
function word: a word that is uninflected and serves a grammatical function but has little identifiable meaning

Other hypernyms of pronoun include closed-class word.

pronoun (noun)	function word (noun)
a function word that is used in place of a noun or noun phrase	a word that is uninflected and serves a grammatical function but has little identifiable meaning