1. Stop the VM you want to resize. Then putty to Proxmox.
  2. We want vm-103 to resize from 20Gb to 40GB.

    Issue the lvextend command.

    proxmox:~# lvextend -L40G /dev/disk1/vm-103-disk-1
      Extending logical volume vm-103-disk-1 to 40.00 GB
      Logical volume vm-103-disk-1 successfully resized

  3. Start VM 103. Go to Disk Management and extend the volume.

  4. Open My Computer and you should have your 40GB drive instead of 20GB.