underwear vs skivvies

underwear and skivvies both are nouns.

Word	Noun	Adjective	Verb	Adverb
underwear	Yes	No	No	No
skivvies	Yes	No	No	No

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

underwear: undergarment worn next to the skin and under the outer garments
skivvies: (used in the plural) men's underwear consisting of cotton undershirt and underpants

Other hyponyms of underwear include BVD's, BVD, intimate apparel, lingerie, long johns, tighty whitey.

underwear (noun)	skivvies (noun)
undergarment worn next to the skin and under the outer garments	(used in the plural) men's underwear consisting of cotton undershirt and underpants