Map a virtual serial port on the guest(ttyS0) to a pseudo terminal(pty) on the host This has to be added to /etc/pve/qemu-server/VMID.conf:
args: -device isa-serial,chardev=myChardevice,id=s -chardev pty,id=myChardevice
After starting the VM check you will see a message:
char device redirected to /dev/pts/7 (label myChardevice)
Inside the guest, you should see now the serial port: