A control or panel in Prism would usually be a view. A view is a UI element that composes a Composite Application and modules usually have one or more views. For a detailed description about modules
and views you can check:
Modularity (design concept)
UI Composition (design concept)
Regarding the sharing of a view, there is no way to make this happen because as you said WPF and Silverlight use different runtimes. The reference implementation uses the
Project Linker: Synchronization Tool to share files. The tool does not share files that fulfill at least one
of the following conditions:
Files that have the file name suffixes .Silverlight.cs or .WPF.cs
Files that reside in Silverlight or Desktop folders
Therefore, you should be able to share almost entire modules, except their view. This of course will also depend in the content of the classes you share.
Because of the differences between the XAML in Silverlight and WPF, the following elements are harder to reuse:
Complex views (XAML)
For a detailed description about sharing code between Silverlight and desktop applications you can check:
Please let me know if this helps.