falsetto vs head voice

falsetto and head voice both are nouns.

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

As nouns, head voice is a hypernym of falsetto; that is, head voice is a word with a broader meaning than falsetto:

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

Other hypernyms of falsetto include head register, head tone.

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

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