Read part 1 from this new series by Bob Familiar at Bob's Blog
Windows 8 also introduces a new type of app called a Metro Style App for developers that wish to make their app available in the Windows 8 Store, for free or for sale. Using Visual Studio 2012, you have a language choice of C++, XAML with C#, VB or C++, or HTML5/JS to create a Metro Style App.
Using the XNA Framework is not a choice for building a Metro Style App. Official Microsoft guidance on game development is documented here. The recommended way to build highly immersive games on Windows 8 is to use HTML5/JS, XAML/C#, XAML/VB or C++ and DirectX, all great choices. But if you have been developing with XNA and have an existing code base, your only option it would seem is running as a desktop app.
This is where MonoGame comes in…