severity vs difficulty

severity and difficulty both are nouns.

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

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

severity: 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 severity include difficultness.

severity (noun)	difficulty (noun)
excessive sternness	an effort that is inconvenient
used of the degree of something undesirable e.g. pain or weather	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
extreme plainness