Proxmox uses noVNC by default. It means copy-and-paste function is not available for your VMs on your Proxmox browser console. To enable copy and function for your Ubuntu VM, you may need other remote tools such as TeamViewer or Anydesk. If you don’t want to install additional software, the simplest solution is to enable xterm.js on your Proxmox browser console instead of using SSH. This article will show you how to enable xterm.js step-by-step.
Step 1: Go to your node and open a terminal shell 
Add a virtual serial port to your VM by
qm set your_vm_id -serial0 socket
Replace your_vm_id with your VM ID. In my case, it is
qm set 100 -serial0 socket
Or you may add it under your VM’s Hardware section
Step 2: Log in to your VM and configure the /etc/default/grub file.
sudo nano /etc/default/grub
Replace the line GRUB_CMDLINE_LINUX_DEFAULT=… with
GRUB_CMDLINE_LINUX_DEFAULT="quiet console=tty0 console=ttyS0,115200"
Press Ctrl + X to save the file.
Step 3: Update grub by
sudo update-grub
Step 4: Shutdown and Start your VM again (If you don’t shut down your VM first, the virtual serial port won’t be attached to your VM). xTerm.js should be available under the console session.
