lover vs sweetheart

lover and sweetheart both are nouns.

Word	Noun	Adjective	Verb	Adverb
lover	Yes	No	No	No
sweetheart	Yes	Yes	No	No

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

lover (noun)	sweetheart (noun)
a person who loves someone or is loved by someone	a person loved by another person
an ardent follower and admirer	a very attractive or seductive looking woman
a significant other to whom you are not related by marriage	any well-liked individual
	a beloved person; used as terms of endearment

lover (adjective)	sweetheart (adjective)
	privileged treatment of a favored person or corporation (sometimes unethically)