L4Linux is a variant of the Linux kernel that is being altered to the extent that it can run virtualized on the L4. L4 is a microkernel and the L4Linux kernel runs a service. L4Linux is not a fork but a variant and is binary compatible with the Linux/x86-Kern, thus it can replace the Linux kernel of any Linux distribution.
L4Linux also allows to set up a virtualized environment vaguely similar to Xen or KVM, but there are a couple of grave differences between the intention of Xen and L4Linux.