upgrade question

Hello.

I'm running a intel x86_64 host with kvm. There are two guests, a linux x86_64 guest and a windows xp guest.
This has been running many years perfectly. Now it's time to upgrade all the software on both the host and linux guest as the OS reached EOL.

At present:
host:

# cat /etc/issue
Ubuntu 10.04.4 LTS \n \l
# uname -rms
Linux 2.6.32-74-server x86_64
# dpkg -l | grep qemu-kvm
ii  qemu-kvm  0.12.3+noroms-0ubuntu9.26

guest:

# cat /etc/issue
Ubuntu 10.04.4 LTS \n \l
# uname -rms
Linux 2.6.32-74-server x86_64

To put it short, I'd like to upgrade to ubuntu 12.04 and after that, maybe also to 14.04.

I'd like advice from the experts on how to perform the upgrade of the KVM guest and hosts, of course after taking a full backup.. :-)

- what should be upgraded first? host or guest?
- are there any caveats?

Any hints or advices are welcome.

Thank you and best regards.
Robi