Recently Xamarin Forms has been expanded to support Windows Phone 8.1 and Windows 8.1. There are instructions online for adding a Windows Phone 8.1 app to your solution and plugging it all together here:-
However there is a small omission which will lead to a build error – #6 tells you to remove the PhonePage base class from your MainPage definition but you actually need to replace it with:-
public sealed partial class MainPage : Xamarin.Forms.Platform.WinRT.WindowsPhonePage
WindowsPhonePage contains the LoadApplication method which you add in #7.
The instructions for Windows 8.1 require the same tweak except using WindowsPage instead of WindowsPhonePage