lover vs kept woman

lover and kept woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
lover	Yes	No	No	No
kept woman	Yes	No	No	No

As nouns, kept woman is a hyponym of lover; that is, kept woman is a word with a more specific, narrower meaning than lover:

lover: a significant other to whom you are not related by marriage
kept woman: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man

Other hyponyms of lover include fancy man, paramour, man, fancy woman, mistress.

lover (noun)	kept woman (noun)
a person who loves someone or is loved by someone	an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
an ardent follower and admirer
a significant other to whom you are not related by marriage