I am looking for a Server oriented distro that we can expect have decent support but also offer as much as possible some of the latest features that KVM might offer.
I am leaning towards Ubuntu LTS 10.04, because well it's LTS and more bleeding edge, but I find Ubuntu not serious enough in terms of support (I say this a heavy Ubuntu user). Given that Centos 6 is not out yet, I am not sure if going Centos 5 would be the best option in terms of getting more features from KVM.
Any other distro you would recommend that could meet the criteria of long term support? (At least 4 years)
Debian 'squeeze' was released on 6th February and includes kvm/qemu 0.12.5, libvirt 0.8.3, virt-manager 0.8.4, making it the most recent stable release and carrying the most recent relevant software of the three (Debian, Ubuntu, CentOS).
The next LTS release of ubuntu is due in 12.04. The last estimate for CentOS 6 I heard was 6 months after RHAS 6, which would mean 1-2 months time, but I've no idea how accurate that is.
I would therefore pick Debian 'squeeze' or RHAS 6, if you are prepared to pay. You will then want to backport newer KVM/libvirt/virt-manager versions in the future, if you are determined to sacrifice stability for features. RHAS will most likely backport the major ones into their product, and support it. Debian will most likely provide recent versions of the software in their backports repository.
If you've already chosen to use KVM, surely using RHEL/CentOS is the best as Redhat owns it?
RHEL is the obvious choice, especially since KVM is being primarily developed for it, and tested on it. If you want the latest and greatest, then Fedora, but that would not provide the LTS you're after.
As of today, I would chose Fedora 23 Server.
You could try scientific linux as the version 6 is already out (clone of RHEL6).