I needed to expand the root partition in a KVM virtual on a debian host server that has KVM and Libvirt install. The virtual disk that this KVM is using has 2 partitions:

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1              63     1060289      530113+  82  Linux swap / Solaris
/dev/vda2         1060290   209715199   104327455   83  Linux

That is the root partition (vda2) is after the swap partition. 

Note: All the steps below are done at the Host level

Expand the LVM Partition

The virtual disk vda is actually a LVM Partition on the host machine. It is very easy to expand the Logical volume:

lvresize -L +20G /dev/mapper/kvm-virtual

Expand the LVM virtual in pool kvm by 20Gb

Resize the block device

But this expansion does not expand the block device - so as far as the virtual is concern - the disk is still 20G smaller. So..

virsh blockresize --path /dev/kvm/virtual --size xxG vID

Where vID is the ID of the KVM (e.g. virtual) and xxG is the new size of the virtual disk

Resize the virtual root primary partition

We now need to resize the root primary partition (/dev/vda2 above) to take the free space. We really need to do this with the virtual not running (i.e. /dev/vda2 not mounted). We could run the virtual with a recue CD image to do this but it can be done entirely in the host.So we need to stop the virtual (virsh shutdown vID)

Make the virtual disk structure visible in the host

We need to make the virtual disk structure visible at the host level. To do this, we use kpartx

kpartx -a /dev/kvm/virtual

This will mapped the LV partition virtual to /dev/mapper:e.g.

kvm-virtual kvm-virtualp1 kvm-virtualp2

Edit the virtual disk using a partition editor

Use a partition editor to resize the root partition. All I had was fdisk - so that what I used:

fdisk /dev/mapper/kvm-virtual

Fdisk does not have a partition resizer. So what you need to do is delete the partition and recreate it. Some other editors has a resize function

Notes:

• you need to make sure that the start sector is the same in the old and new partition (The default value may not be)
• You can only easily expand the last partion
• When you exit the editor - you need to refresh the partitions using kpartx so that the host knows the changes:
                 kpartx -uv /dev/kvm/virtual

Resize the filesystem 

You now need to resize the filesystem. In my case, the filesystem was ext4 so I did the following

e2fsck -f /dev/mapper/kvm-virtualp2
resizefs /dev/mapper/kvm-virtualp2

Start the virtual

We now need to remove the partition information on the host:

kpartx -dv /dev/kvm/virtual

We are now ready to start the virtual

virsh start virtual