If you have Video Saver installed and working then you should be able to use the Select files on the computer, or similar depending on the version you have, option to Import the MXV files and use the Save movie on computer option to export to MPEG-4 (MP4).
Alternatively if you have a full version of Magix Photostory, Movie Edit Pro, Magix Movie Studio 18 or later, Video Pro X installed - any of these can import the MXV files and export to MP4.
Note:
If you do not have any of these products installed the trial versions will not be of help to you as they are limited to exporting 3 mins of video.
The MXV file is not compatible with video editors other than those I mentioned above