rigor vs difficulty

rigor and difficulty both are nouns.

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

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

rigor: 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 rigor include difficultness.

rigor (noun)	difficulty (noun)
something hard to endure	an effort that is inconvenient
the quality of being valid and rigorous	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
excessive sternness