diver vs pearl diver

diver and pearl diver both are nouns.

Word	Noun	Adjective	Verb	Adverb
diver	Yes	No	No	No
pearl diver	Yes	No	No	No

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

Other hyponyms of diver include deep-sea diver, pearler, scuba diver.

diver (noun)	pearl diver (noun)
large somewhat primitive fish-eating diving bird of the Northern Hemisphere having webbed feet placed far back; related to the grebes	a diver who searches for molluscs containing pearls
someone who dives (into water)
someone who works underwater