Is there any difference between the two? Are they interchangeable? Both mean "although/even though."
You use とはいえ to partially deny, or say something contrary to, what was stated earlier while accepting it on the whole.
Although I passed the exam, I still have more to study.
(While it is true that I passed the exam, I still have more to study.)
On the other hand, のに is used to state two contrasting or contradictory facts. In addition, it carries a sense of unexpectedness or surprise on the part of the speaker.
Although I passed the exam, I am still forced to study.
(Despite the fact that I passed the exam, I am still forced to study.)
I would say とはいえ is closer in meaning to けど or けれども than to のに in that it lacks this sense of unexpectedness or surprise.
Whenever I encounter とはいえ it always strikes me as having a strong feeling of one can/could say. The effect is much like のに. Also, I suspect (again nonnative speaker here), that とはいえ is not generally used in everyday conversation, but has become more of a literary form. It also feels a bit more objective to me than のに. Native speakers (or otherwise knowledgeable folk) please correct me lest I mislead