Xamarin is a set of tools and libraries for building cross-platform mobile apps on the .NET platform.
Build native apps for iOS, Android, Windows, and macOS on a shared .NET codebase.
Share an average of 75% of app code across all platforms.
UWP extends the .NET platform to enable development for any Windows 10 device—PC, tablet, phone, Xbox, HoloLens, Surface Hub, and Windows 10 IoT Core.
UWP provides a UI framework for writing responsive apps that support a number of screen sizes and interaction models, such as touch, mouse and keyboard, game controller, and pen.
Extension SDKs offer specialized APIs for each device class, like the holographic capabilities of HoloLens.