difficulty vs severity

difficulty and severity both are nouns.

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

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

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

difficulty (noun)	severity (noun)
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	used of the degree of something undesirable e.g. pain or weather
	something hard to endure
	extreme plainness