plainness vs severeness

plainness and severeness both are nouns.

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

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

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

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