The built in xmltv grabber for tvheadend can point any folder you want...in this case, you have it pointing to ~/.xmltv/xmltv.xml
Usually it's just easy to set that up through the kodi interface - but I get the feeling that you don't have kodi setup on this debian install? Otherwise you would have likely just installed my whole addon.
My experience on systems other than LibreELEC is limited - so someone else may have to chime in here...but if you install the zap2epg addon in that same folder - ~/.xmltv/ - zap2epg should be able to generate the xmltv.xml file there and then tvh will read it.
A little more information on what you are trying to setup may help. Is this being setup as just a backend system that will be accessed from other frontend systems? Is it a standalone setup - RPi or other? Do you plan to use Kodi as your interface for playback?
If I get an idea of what you're trying to configure, I can probably provide a clearer set of setup steps.