The only way to build games on Windows 8 and publish it on Windows Store is to use HTML5/JS, XAML/C#, XAML/VB or C++ and DirectX. But if you already have a XNA game codebase, you need not rewrite your whole codebase.
Windows Phone Game developers can now easily port their XNA games to Windows 8 Metro environment using MonoGame. MonoGame is an Open Source implementation of the Microsoft XNA 4 Framework. It allows XNA developers on Xbox 360, Windows & Windows Phone to port their games to the iOS, Android, Mac OS X, Linux and Windows 8 Metro.
Read more about it here.