well, to really analyse it, we'd have to break down 'passing' into its constituent parts.
In terms of 'raw ability', being able to pass a ball accurately, with the right pace, curl etc. I think messi is better. He pulls off through balls that, in addition to great vision, are tremendously difficult to execute with the proper weight, timing and technique.
But then, this has never really been Xavi's strongest suite. He has always been behind the likes of Pirlo when it comes to executing the most difficult of passes and rarely attempts them.
Where Xavi wins out is the game in his head and unbelievable consistency. He always picks the right pass, is impossible to harass and harry into giving the ball away and his ability to make so many simple passes that connect into influent play for the team overall is unparalleled by anyone anywhere. No matter the opposition or the game, Xavi will sit in midfield and distribute play without error.
Furthermore, while he may not be the best at the 10/10 passes, he retains his amazing consistency even on 8/10 passes, which is a tremendous feat in and of itself as well and helps him regularly top the assist charts.
Messi isn't in Xavi's league in those departments (no one is), but in addition to having better 'raw' passing ability, I think Messi does have better vision. He's better at spotting the pass no one else would notice.