For those wondering about the modern performance cost of using KVM on Ubuntu Linux for virtualizing a guest OS, here are some simple benchmarks comparing Ubuntu 14.10 in its current development stage with the Linux 3.16 versus running the same software stack while virtualized with KVM and using virt-manager.
Originally I intended for this testing to be an extensive look at the various virtualization options for Linux (similar to past tests comparing VMware, VirtualBox, Xen, KVM, etc) but this comparison was cut short once running into problems with Xen. When booting the Xen-enabled Linux 3.16 kernel on this Core i7 4790K Devil's Canyon system, there were show-stopping issues encountered.
As a result, this comparison is just benchmarks of Ubuntu 14.10 running on the Core i7 4790K system "bare metal" and then again when using KVM and setting up the Ubuntu 14.10 guest using virt-manager. The default settings were used and the guest installation was allowed to access all eight CPU threads and 13GB of the system's 16GB of memory, while all other virtualization settings remained at their defaults. The KVM guest was the only active process running on the system at the time of testing.
All of the Ubuntu Linux performance benchmarks were carried out via the Phoronix Test Suite open-source benchmarking software. A more thorough Linux virtualization comparison will be attempted when Ubuntu 14.10 is closer to its October release and from a system where Xen will work without faults.