.AXML has no relation to XAML at all. It's just plain Android layouts: when doing plain Java Android dev, they're just .xml files. I believe that Xamarin made up a new extension for them because it allows them to easily associate the file extension with a special editor in the IDE (Visual Studio and Xamarin Studio/MonoDevelop).
Xamarin's Android interface builder is actually remarkably decent.
It looks like Xamarin uses .AXML for Android (maybe a XAML flavor) but has to adhere to Interface Builder for iOS (http://docs.xamarin.com/guides/cross-platform/application_fu..., specifically the Visual Designer section). XAML likely won't gain any traction on those platforms though due to NiH on all sides. HTML/CSS is likely as close as we get for now but that isn't a "native view" in any of the platforms mentioned.