Check out https://help.ubuntu.com/community/KVM for more updated and complete information on installing KVM on Ubuntu.
kvm is qemu plus some great kernel level stuff for VT enabled CPUs.
Some rough notes:-
How to tell if you can use kvm
grep vmx /proc/cpuinfo
However, if you have an Intel CPU it's possible that your computer manufacturer may have turned off VT in the BIOS. See below for more info.
grep svm /proc/cpuinfo
On some Gigabyte motherboards it is turned off in the BIOS if you use "Load safe defaults" (GiBy M61SME-S2 nF405 RLVSM bios < F7)
If you see something you have it, if you don't you don't.
sudo apt-get install kvm qemu
Load the modules
sudo modprobe kvm
Load Intel module
If you have an Intel CPU
sudo modprobe kvm-intel
Note if you get this when you load the module then it may mean VT is switched off in the BIOS.
$ sudo modprobe kvm-intel FATAL: Error inserting kvm_intel (/lib/modules/2.6.20-15-generic/kernel/drivers/kvm/kvm-intel.ko): Operation not supported
Typing dmesg you may find the following at the end:-
kvm: disabled by bios
You might need to enable the virtualization setting in the BIOS. Each BIOS is different and it can be difficult to find (it may be in the "Advanced" section or under "CPU" or "Security"). You may also need to power shutdown for the changes to take effect (some bios will do this automatically).
Load AMD module
If you have an AMD CPU
sudo modprobe kvm-amd
Add yourself to the group
sudo addgroup `whoami` kvm
Create a KVM disk image
This example makes a virtual disk that grows on the fly:
qemu-img create -f qcow2 virtualdisk.img 10G
Start the emulator
kvm -hda virtualdisk.img -cdrom cdrom.iso -m 256 -net nic -net user -soundhw all
Load the modules more permanently
Once you know it all works, edit /etc/modules and add the following two lines