We've had several KVM hosts up and running on Ubuntu 9.04 and 9.10 for the last 6+ months with some minor issues while initially setting up guests, but none since. Our hosts average 3 guests each and the guests do lots of different things, from multi-homed VPN servers to NAS systems with direct access to hard disks. The performance and ease of setup (once the learning curve was out of the way) has been transformative for our network.
Migration has been very easy through VNC connections over ssh via a virt-manager client; cloning has not been as smooth, as new instances have had a tendency towards flaky network errors.
The only other ongoing complaint I have is that it would be nice to have support for QEMU VLANs as an option in virt-install so we could avoid having to use the more tedious KVM commands directly. Other than that, we've been extremely happy, and most of the manangement tools are shared with Xen so I'd say use whichever one works best for you.
Qemu:
QEmu is a complete and standalone software of its own. You use it to emulate machines, it is very flexible and portable. Mainly it works by a special 'recompiler' that transforms binary code written for a given processor into another one (say, to run MIPS code on a PPC mac, or ARM in an x86 PC).
To emulate more than just the processor, Qemu includes a long list of peripheral emulators: disk, network, VGA, PCI, USB, serial/parallel ports, etc.
KQemu:
In the specific case where both source and target are the same architecture (like the common case of x86 on x86), it still has to parse the code to remove any 'privileged instructions' and replace them with context switches. To make it as efficient as possible on x86 Linux, there's a kernel module called KQemu that handles this.
Being a kernel module, KQemu is able to execute most code unchanged, replacing only the lowest-level ring0-only instructions. In that case, userspace Qemu still allocates all the RAM for the emulated machine, and loads the code. The difference is that instead of recompiling the code, it calls KQemu to scan/patch/execute it. All the peripheral hardware emulation is done in Qemu.
This is a lot faster than plain Qemu because most code is unchanged, but still has to transform ring0 code (most of the code in the VM's kernel), so performance still suffers.
KVM:
KVM is a couple of things: first it is a Linux kernel module—now included in mainline—that switches the processor into a new 'guest' state. The guest state has its own set of ring states, but privileged ring0 instructions fall back to the hypervisor code. Since it is a new processor mode of execution, the code doesn't have to be modified in any way.
Apart from the processor state switching, the kernel module also handles a few low-level parts of the emulation like the MMU registers (used to handle VM) and some parts of the PCI emulated hardware.
Second, KVM is a fork of the Qemu executable. Both teams work actively to keep differences at a minimum, and there are advances in reducing it. Eventually, the goal is that Qemu should work anywhere, and if a KVM kernel module is available, it could be automatically used. But for the foreseeable future, the Qemu team focuses on hardware emulation and portability, while KVM folks focus on the kernel module (sometimes moving small parts of the emulation there, if it improves performance), and interfacing with the rest of the userspace code.
The kvm-qemu executable works like normal Qemu: allocates RAM, loads the code, and instead of recompiling it, or calling KQemu, it spawns a thread (this is important). The thread calls the KVM kernel module to switch to guest mode and proceeds to execute the VM code. On a privileged instruction, it switches back to the KVM kernel module, which, if necessary, signals the Qemu thread to handle most of the hardware emulation.
One of the nice things of this architecture is that the guest code is emulated in a posix thread which you can manage with normal Linux tools. If you want a VM with 2 or 4 cores, kvm-qemu creates 2 or 4 threads, each of them calls the KVM kernel module to start executing. The concurrency—if you have enough real cores—or scheduling—if not—is managed by the normal Linux scheduler, keeping code small and surprises limited.
Best Answer
On google you can find a lot of website who talk about the performance of differents filesystem with KVM.
Take a look at this one : ZFS, BTRFS, XFS, EXT4 and LVM with KVM – a storage performance comparison
According to the author Gionatan Danti :
He conclude by :
Another blog talk about BTRFS, you can read on a lot of forums that Copy On Write (COW) need to be disable for getting better performances with KVM.
Chris Irwin talk about the benefits of BTRFS and talk about an alternative :
link : live with btrfs
Another way to know if it's okay for your use is testing by yourself if the performance is good and if it is reliable without copy on write.
If BTRFS is not the best for you, you can try ZFS. You have the same Backup fonctionality and lot of other improvements but it a bit tricky to implement in linux.