It looks like that both .mat and .prv files are related and used in Vue nature 3D rendering tool, so if any conversion between mat and prv is possible, it would be done with Vue. Sadly, we couldn't really verify whenever this conversion is actually useful, or even possible.
Convert Vue material file to E-on Vue preview picture file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert mat to prv on Windows
An advanced software for rendering natural 3D environments
Convert from mat file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.