party girl vs young woman

party girl and young woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
party girl	Yes	No	No	No
young woman	Yes	No	No	No

As nouns, young woman is a hypernym of party girl; that is, young woman is a word with a broader meaning than party girl:

party girl: an attractive young woman hired to attend parties and entertain men
young woman: a young female

Other hypernyms of party girl include fille, girl, miss, missy, young lady.

party girl (noun)	young woman (noun)
an attractive young woman hired to attend parties and entertain men	a young female