It’s true that Xamarin feels like more than just a mobile framework but, at its heart, that’s precisely what it is: a tool to build Android, iOS, and UWP apps in C#. This is a highly-reliable platform that provides you with native performance supported by Microsoft technology and with enough features to control your resulting app. 

Mono is a free and open-source .NET Framework-compatible software framework. Originally by Ximian, it was later acquired by Novell, and is now being led by Xamarin, a subsidiary of Microsoft and the .NET Foundation.