plainness vs severity

plainness and severity both are nouns.

Word	Noun	Adjective	Verb	Adverb
plainness	Yes	No	No	No
severity	Yes	No	No	No

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

Other hyponyms of plainness include chasteness, restraint, simpleness, simplicity, austereness, severeness, bareness, starkness.

plainness (noun)	severity (noun)
the appearance of being plain and unpretentious	excessive sternness
an appearance that is not attractive or beautiful	used of the degree of something undesirable e.g. pain or weather
clarity as a consequence of being perspicuous	something hard to endure
the state of being unmixed with other material	extreme plainness