severeness vs difficulty

severeness and difficulty both are nouns.

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

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

severeness: something hard to endure
difficulty: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of severeness include difficultness.

severeness (noun)	difficulty (noun)
extreme plainness	an effort that is inconvenient
excessive sternness	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
something hard to endure
used of the degree of something undesirable e.g. pain or weather