cave vs cavern

cave and cavern both are nouns.

Word	Noun	Adjective	Verb	Adverb
cave	Yes	No	Yes	No
cavern	Yes	No	Yes	No

As nouns, cavern is a hyponym of cave; that is, cavern is a word with a more specific, narrower meaning than cave:

cave: a geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea
cavern: a large cave or a large chamber in a cave

Other hyponyms of cave include cove, grot, grotto.

cave (noun)	cavern (noun)
a geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea	a large cave or a large chamber in a cave
	any large dark enclosed space

cave (verb)	cavern (verb)
explore natural caves	hollow out as if making a cavern
hollow out as if making a cave or opening