KWin is the window manager for the KDE Plasma desktop environment, and Xwayland is a compatibility layer that allows X11 applications to run on Wayland. When using KWin on a Wayland session, Xwayland enables the use of applications that have not yet been ported to Wayland.
Would you like to know how to enable or configure KWin with Xwayland, or do you have a specific issue you're encountering?