head voice vs falsetto

head voice and falsetto both are nouns.

Word	Noun	Adjective	Verb	Adverb
head voice	Yes	No	No	No
falsetto	Yes	Yes	No	No

As nouns, falsetto is a hyponym of head voice; that is, falsetto is a word with a more specific, narrower meaning than head voice:

head voice: the higher ranges of the voice in speaking or singing; the vibrations of sung notes are felt in the head
falsetto: a male singing voice with artificially high tones in an upper register

head voice (noun)	falsetto (noun)
the higher ranges of the voice in speaking or singing; the vibrations of sung notes are felt in the head	a male singing voice with artificially high tones in an upper register

head voice (adjective)	falsetto (adjective)
	artificially high; above the normal voice range