Windows Apps Team recently announced XAML Standard, a specification of XAML that will be applicable across all XAML based technologies such as UWP, and Xamarin.Forms.
From the blog post:
XAML Standard is a specification that defines a standard XAML vocabulary. With that vocabulary, frameworks that support XAML Standard can share common XAML-based UI definitions. The goal is for the first version, XAML Standard 1.0, to be available later this year.
Post-specification plans include support of XAML standard in Xamarin.Forms and UWP. You can continue developing your UWP and Xamarin.Forms apps as you do today. When XAML Standard support is enabled, you will be able to reuse and share between the frameworks and expand into more platforms.
We are at the beginning of a journey that makes it easy for you to reuse your XAML source files between some simple Xamarin.Forms and UWP views. For example – a Settings.xaml page, where you typically have some text, toggle switches and some buttons. You’d only need to design and create one XAML file to describe this UI and that can be used everywhere.
Nothing changes for existing developers – you can continue to use the same APIs you have always used in both frameworks. XAML Standard will help you reuse/share any common UI code that you wish to share between end points.
You can download XAML Standard here:
https://github.com/Microsoft/xaml-standard