valentine vs sweetheart

valentine and sweetheart both are nouns.

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

As nouns, sweetheart is a hypernym of valentine; that is, sweetheart is a word with a broader meaning than valentine:

valentine: a sweetheart chosen to receive a greeting on Saint Valentine's Day
sweetheart: a person loved by another person

Other hypernyms of valentine include steady, sweetie, truelove.

valentine (noun)	sweetheart (noun)
a sweetheart chosen to receive a greeting on Saint Valentine's Day	a person loved by another person
	a very attractive or seductive looking woman
	any well-liked individual
	a beloved person; used as terms of endearment

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