What tense is "would have been"?

the verb group 'would have been' is

present (have), finite, perfect (been), non-progressive, modal (would), positive, non-contrastive

which in so-called 3rd conditionals expresses past irrealis, eg,

Had you been there, it would have been nice.

In English, tense of a finite clause is always marked on the first verb of the finite clause, and is either 'present' or 'past'. (A non-finite clause doesn't have any tense.)

In your example, the first verb would is the only finite verb--and the only tensed verb--and is in the past tense. Therefore, would have been is in the past tense.