valley vs hollow

valley and hollow both are nouns.

Word	Noun	Adjective	Verb	Adverb
valley	Yes	No	No	No
hollow	Yes	Yes	Yes	No

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

valley: a long depression in the surface of the land that usually contains a river
hollow: a small valley between mountains

Other hyponyms of valley include dale, glen, gully, holler, nullah, ravine, rift valley.

valley (noun)	hollow (noun)
a long depression in the surface of the land that usually contains a river	a depression hollowed out of solid matter
	a small valley between mountains
	a cavity or space in something

valley (adjective)	hollow (adjective)
	not solid; having a space or gap or cavity
	lacking in substance or character
	devoid of significance or force
	as if echoing in a hollow space

valley (verb)	hollow (verb)
	remove the interior of
	remove the inner part or the core of