belle vs young woman

belle and young woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
belle	Yes	No	No	No
young woman	Yes	No	No	No

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

belle: a young woman who is the most charming and beautiful of several rivals
young woman: a young female

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

belle (noun)	young woman (noun)
a young woman who is the most charming and beautiful of several rivals	a young female