OK, I just do some search, and find below information is good to explain XDVD: Introduction of the XDVD (eXtended DVD) format
A new format called XDVD has been developed based on DVD-Video standard, to allow longer hours. Though the length of GOP(Group of Pictures) is limited to 18 frames in the DVD-Video standard, concerning the XDVD format, one GOP can contain up to 60 frames! High-quality and high-compression can be realized in the same time. For instance, it is possible with that format to put 10 hours of video in a 4.7 GB DVD !!!