You can still use XNA for windows phone 8 but it has to be targeted to wp 7.x. You could also use monogame which you code in XNA and it will convert it into pretty much any platform you want even ios/android/windows 8/wp8. Or you could learn how to use Unity3D which they just released a beta compiler for WP8.
Microsoft also released a helper class for Visual Studio 2012 to make it more "xna" like for wp8 development.
So basically....
Stick to XNA but compile for wp7.x
Switch to MonoGame which uses XNA and you can then compile to multiple platforms and not just windows phone
Switch to Unity3D that also compiles to multiple platforms, but does not use XNA, it uses its own engine and you get a designer for free.
Learn the new way of coding for WP8 using the helper class.