You don’t really link your Xbox account to your Origin/EA account. Rather you link your Xbox gamertag, which is only part of your Xbox account, to your Origin/EA account. Your Xbox gamertag is called a persona. So, technically, you are linking your persona to your Origin/EA account.
If you need information on this, then look for the post entitled “Linking Your Persona To Your Origin / EA Account (version 2)”. This post was made on Wednesday, February 22, 2012, in the following thread.