quite obvious actually...Xavi was the best player in the UEFA Euro 2008, before he became really famous, Xabi Alonso was not.
Xavi was almost nominated for the Ballon D'Or in 2010, Alonso was not.
Xavi has one of the best visions in the world, Xabi Alonso only has half of his vision to be honest.
free-kick taking: both are pretty equal
Passer: Xavi is 10x better passer than Xabi Alonso is
goal scorer: equal
defensive midfielder: Alonso
playmaker: slightly Xavi
Xavi has much, much better leadership too, can easily take the role as permanent captain if they allowed him.
its quite hilarious how Madridistas try to compare their midfielders such as Guti and Xabi Alonso with Xavi but none come close, only Zidane is better than Xavi.