onion vs pearl onion

onion and pearl onion both are nouns.

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

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

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

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