PVE stands for Proxmox Virtual Environment (similar to ESXi). I also installed ROCK on PVE and this works really well.
“I have installed ROCK on PVE, How to navigate to [Roon’s Database Location], and where is RoonServer folder.”
If you want full control then you should use a (basic) Linux install and install Roon Server on top of that. The only granted access in ROCK is to the “data” share where you can put your codecs files.
See also:
Edit: Ok, it looks like the Data share also gives you acces to some more files.