pearl onion vs onion

pearl onion and onion both are nouns.

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

As nouns, onion is a hypernym of pearl onion; that is, onion is a word with a broader meaning than pearl onion:

pearl onion: a small onion with a sweeter taste than that of a common onion.
onion: an aromatic flavorful vegetable

pearl onion (noun)	onion (noun)
a small onion with a sweeter taste than that of a common onion.	an aromatic flavorful vegetable
	bulbous plant having hollow leaves cultivated worldwide for its rounded edible bulb
	the bulb of an onion plant