Xv format is a unique virtual format of Thunder, that is, cache file. When you watch it with Thunder again, you can watch it without buffering. This is the only advantage. You can't watch it offline, because this video is digitally signed. If you want to play, you need to sign online for verification.

As for the xv file you said when watching videos online, that's because xv is a cache file and must be stored in your computer, so watching videos will not get stuck. There is no solution. You can only watch the video, and then find the XV file to delete.