heads-up vs warning

heads-up and warning both are nouns.

Word	Noun	Adjective	Verb	Adverb
heads-up	Yes	Yes	No	No
warning	Yes	Yes	No	No

As nouns, warning is a hypernym of heads-up; that is, warning is a word with a broader meaning than heads-up:

heads-up (noun)	warning (noun)
a warning message	a message informing of danger
	cautionary advice about something imminent (especially imminent danger or other unpleasantness)
	notification of something, usually in advance

heads-up (adjective)	warning (adjective)
fully alert and watchful	serving to warn