What is the fastest Virtualization solution for Linux?

Fastest under what conditions? With hardware virtualization, the speed should be identical on all virtualization platforms.

Therefore the only thing that you should consider looking for is hardware virtualization support in the software.

As far as I know, Virtualbox doesn't support IOMMU hardware virtualization yet. KVM, VmWare and Xen should. Xen and VmWare should be the only ones supporting IOMMU on graphic cards (with differing degrees of success).

The fastest solution is generally the one that introduce the less overhead compared to a non virtualized environment. If you can cope with its "non OS diversity" limitation, that would be an OS level virtualization implementation. With Linux, that translates to OpenVZ/Virtuozzo, Linux containers (lxc) and VServer.