severeness vs plainness

severeness and plainness both are nouns.

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

As nouns, plainness is a hypernym of severeness; that is, plainness is a word with a broader meaning than severeness:

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