The raven continuously repeating the word "nevermore" is an example of what figurative language?

This is a personification. An idiom would be something like a proverb while hyperbole would be exaggeration. A metaphor is when there's a meaning that's implied. Personification is when you give objects that don't behave like people, human traits. The raven in reality probably doesn't say anything, just croaks or whatever, but in the poem he keeps saying it because it's a poem.