party favor vs souvenir

party favor and souvenir both are nouns.

Word	Noun	Adjective	Verb	Adverb
party favor	Yes	No	No	No
souvenir	Yes	No	No	No

As nouns, souvenir is a hypernym of party favor; that is, souvenir is a word with a broader meaning than party favor:

party favor: souvenir consisting of a small gift given to a guest at a party
souvenir: something of sentimental value

Other hypernyms of party favor include keepsake, relic, token.

party favor (noun)	souvenir (noun)
souvenir consisting of a small gift given to a guest at a party	something of sentimental value
	a reminder of past events