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METHOD #1: Partitions are ext2/ext3/ext4 based

The nuts of this method are as follows:

# 1. stop the VM # 2. move the current image mv mykvm.img mykvm.img.bak

# 3. create a new image qemu-img create -f raw addon.raw 30G

# 4. concatenate the 2 images cat mykvm.img.bak addon.raw » mykvm.img

Now with the larger mykvm.img file in hand, boot gparted and extend the existing partition into the newly added disk space. This last step basically extends the OS partition so that it can make use of the extra space.

http://serverfault.com/questions/324281/how-do-you-increase-a-kvm-guests-disk-space