Beginning with Windows Phone 8.1, XAP will be replaced by APPX as the file format used to install apps on the Windows Phone platform, a move which was done by Microsoft in order to unify the app development platforms for Windows Store apps and Windows Phone apps.
Формат xap также используется в Microsoft Silverlight,* Формат XAP заменит CAB в Windows Phone 7* Silverlight 2: structure of the new .XAP file (Silverlight packaged application), Katrien's MSDN Blog* Инструкция по установке приложений в Windows Phone 7