folk metal vs heavy metal music

folk metal and heavy metal music both are nouns.

Word	Noun	Adjective	Verb	Adverb
folk metal	Yes	No	No	No
heavy metal music	Yes	No	No	No

As nouns, heavy metal music is a hypernym of folk metal; that is, heavy metal music is a word with a broader meaning than folk metal:

folk metal: a fusion genre of heavy metal music and traditional folk music that developed in Europe during the 1990s.
heavy metal music: loud and harsh sounding rock music with a strong beat; lyrics usually involve violent or fantastic imagery

Other hypernyms of folk metal include ethnic music, folk, folk music, heavy metal.

folk metal (noun)	heavy metal music (noun)
a fusion genre of heavy metal music and traditional folk music that developed in Europe during the 1990s.	loud and harsh sounding rock music with a strong beat; lyrics usually involve violent or fantastic imagery