Short version: You can't. XAP files are never stored on the phone. You're asking the equivalent of "How can I get SkypeSetup.exe out of my C:\Program Files\Skype directory?"
It *might* be possible to reconstruct the XAP, but you'd need to build it from the application's install directory. Currently the only phones that you can access this directory on is HTC (and possibly Samsung, when Heathclif releases his tool). I actually already have an app that does this (access the install directory, not rebuild the XAP) and have avoided even trying to rebuild the XAP as I have more than enough other things to do and could see no real value in doing the requild. If you can convince me otherwise, I'll listen. "So I can copy the app to my friend's phone" isn't going to fly, though; I'll tell you that now. Requests like that get reported to the mods. This forum is *NOT* about app piracy; read the stickies.