kept woman vs lover

kept woman and lover both are nouns.

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

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

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

Other hypernyms of kept woman include adult female, woman.

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