Hey, they're not even vaguely similar. KendoUI is a JavaScript framework providing cross-platform capabilities of web apps on mobile, or by combining with mobile SDKs like MoSync, for producing apps. You can probably even use KendoUI with Xamarin. KendoUI will be the UI for your app, and you can code the back end in C# with Xamarin.
This confirms my understanding. I was referred to Kendo by a friend when I told them about Xamarin and I was trying to determine where the products overlapped.