This would be a tough fight. If Luke fought Revan and Yoda fought Exar then I think the Jedi would probably win. Exar's extremely arrogant, and let's his anger and pride cloud his judgement, and while I think he's more powerful than Yoda, this would be like the ROTS Obi vs. Anakin battle with Yoda being Obi-Wan.
I think Yoda would kill Exar before Lord Revan could kill Luke and they would overpower Revan together.