I am running several VMs with kvm. After a little while I noticed that I was a little to stingy with the amount of space assigned to each VM.

These are the steps I used to reassign more space to the VMs:

1. Stop the VM
2. Make a backup!
3. Get the difference between the size at the moment and the wished size (in my case 4 GiB)
4. Create a image with this size consisting of zeros

   # copy 4096 times 1 MB of zeros into raw.img
   dd if=/dev/zero of=raw.img bs=1M count=4096
   

5. Join the image and the zeros into a new file:

   cat vm.img raw.img > new_vm.img
   

6. Get the starting sector of the partition and delete the partition

   gdisk new_vm.img
   p (remember the starting sector!)
   d (use the number of the partition)
   w (write changes to disk, let gdisk correct the GPT header which is not at the end)
   

7. Recreate the partition

   gdisk new_vm.img
   n (use the number and the first sector from above)
   [you might want to set the attributes (like bootable) as well]
   w
   

8. Start the VM and then on (in) the VM and run fsck on the partition

   fsck /dev/mypartition
   # or for root fs:
   touch /forcefsck && reboot
   

9. Grow the partition inside the VM

   resize2fs /dev/mypartition
   

Update 2017-08-15

It is possible to use sparse files for the update as well:

# create sparse file
truncate -s 16G new_vm.img

# copy old image
dd if=vm.img of=new_vm.img conv=notrunc

Then continue with deleting and recreating the partition you want to resize.