Qemu images can’t be growed. In this example I will show you a little hack to grow a 6GiB qcow image to a 10GiB image. Beware: these operations can take a lot of time to perform and require lots of free space.

First, convert your qcow image to a plain raw file: 

1$ qemu-img convert system.qcow -O raw system.raw

Then, create a dummy file (filled with zeros) of the size of extra space you want to add to your image. In this case, 4GiB (=10GiB - 6GiB): 

1$ dd if=/dev/zero of=zeros.raw bs=1024k count=4096

Fearlessly, add your extra space to your raw system image: 

1$ cat system.raw zeros.raw > big10G.raw

After that you can boot qemu to verify that added free space is available: 

1$ qemu -hda big10G.raw

Here is an real case example of what you can see in a qemu image on which Windows XP was installed:

Windows disk management inside QEMU showing the grown 10 GB disk, with the 5.99 GB FAT32 system partition and 4.01 GB still unallocated beside it

Now, to grow your primary partition, I suggest you to download a Live CD like gparted Live CD or System Rescue CD , and boot on the .iso file with qemu: 

1$ qemu -hda big10G.raw -cdrom gparted-livecd-0.3.4-5.iso -boot d

This will allow you to grow and manipulate all your partitions safely thanks to parted and other open source system tools.

Finally you can convert back your raw image to a qcow one to not waste space: 

1$ qemu-img convert big10G.raw -O qcow growed-system.qcow

That’s all!

By the way, I think it’s possible to perform the second and third step of this how-to in a single operation using dd only.

Update : I missed it, but this issue is also described in the FAQ from the unofficial #qemu wiki (look at “ How do I resize a disk image? ” question).