seriousness vs severity

seriousness and severity both are nouns.

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

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

Other hypernyms of seriousness include badness, severeness.

seriousness (noun)	severity (noun)
the trait of being serious	excessive sternness
the quality of arousing fear or distress	used of the degree of something undesirable e.g. pain or weather
an earnest and sincere feeling	something hard to endure
	extreme plainness