In Windows Phone 8.1 the Windows Runtime XAML is available along with improved Windows Runtime support. This convergence between platforms enable Universal Apps that can target both Windows 8.1 and Windows Phone 8.1 while sharing most of the code, including user interface. The Windows Phone 8.1 is still capable of running Silverlight based XAML apps and new features and API were also added to this framework too (called Silverlight 8.1)
The XAML Framework is the successor to Windows Presentation Foundation (WPF) and Silverlight - similar XAML-based UI frameworks used for desktop applications and portable applications respectively. The XAML framework uses a lot of the same names for its APIs as both of these older technologies - especially Silverlight, but its use is limited to the Windows OS (specifically Windows 8 and later) as with WPF. The major difference is that Silverlight and WPF are largely based on the Common Language Runtime (CLR) and as such require using programming languages such as C# or Visual Basic, while the XAML framework is part of the Windows Runtime, written in native code and also available, and has tools for development, with C++ or C++/CX.