As far as I know, if you are using noVNC you cannot simply copy and paste to/from your VM and desktop, whether you are using Windows or Linux inside your VM.
If you need that functionality, you should be able to use either Spice/xterm.js instead of noVNC.
An alternative is to SSH/RDP/VNC into your VM through another means
Containers and the Proxmox host shell should allow you to copy/paste.
I'm suddenly facing this problem - copy and paste just stopped working.... i thought maybe I changed something by accident but I can't find anything... I know the thread is old, but thought maybe others might have had this problem. I didn't do any updates either...