Q. Where does Minecraft Windows 10 edition save the game worlds?
A. Unlike the regular Minecraft which saves the worlds to location %USERPROFILE%\AppData\Roaming\.minecraft\saves\, the Windows 10 edition saves the worlds to %USERPROFILE%\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\
LocalState\games\com.mojang\minecraftWorlds. The worlds save in different formats so you cannot just copy worlds from the java based version to the Windows 10 version.