WinRT Unmanaged

WinRT needs to be lower level than .NET so that it can wrap the Win32 API as well as the new Metro API. It is therefore written in C++ using an enhanced version of COM so that .NET will be able to interface easily with it.

It does have a superficial resemblance to .NET in that the version of C++ which is used for programming Metro apps (C++/CX) has similar syntax to the managed version of C++ (C++/CLI) and also uses a similar metadata format. However, as it's COM based, reference counting is used rather than garbage collection.

.NET apps will continue to run under the control of the CLR and use garbage collection as they do now.

Metro apps written in C++/CX or .NET will both use XAML as their page markup language whereas Javascript will use HTML5. 

It is likely that C++/CX will execute faster than .NET apps which in turn will execute faster than Javascript/HTML5 apps. However, it should be easier to write C#/VB.NET apps than C++/CX apps. 

Whether Javascript apps will be the easiest to write of them all remains to be seen.