Expand the QCOW2 image file at the kvm host os:
qemu-img info ubuntu20.04.qcow2 qemu-img resize ubuntu20.04.qcow2 +5G
From within the guest run the following commands:
sudo pvs
sudo growpart /dev/vda 3
sudo lsblk
sudo pvresize /dev/vda3
sudo vgs
sudo lvextend -r -l +100%FREE /dev/ubuntu-vg/ubuntu-lv
That’s all. The LVM should be expanded to use the new space.