KVM Host Hardware Requirements
Although the requirements below are modest. I have to point out that this is one of those cases where bigger is better.
Check out this article for a more thorough explanation http://linux-kvm.org/
Check for the vmx flag in /proc/cpuinfo
Check for the svm flag in /proc/cpuinfo
The KVM Host itself does not consume much memory. I believe it would be possible to boot the KVM Host with only 256M of RAM. It really depends on the memory requirements of the KVM Guests.
Not much diskspace is needed for the KVM Host. The reference system has a / partition of only 3G. This is really about the size of the KVM Guests. Another consideration is the software raid, which at least requires 2 disks.
At least 1 network interface is required for the system to make any sense at all. In theory it would run fine without a network interface, but using it would be a pain.