Lguest

Lguest is a Linux kernel x86 virtualization hypervisor introduced in kernel version 2.6.23 (released 9 October 2007) and removed in kernel version 4.14 (November 2017). The hypervisor is an operating-system-level virtualization system capable of running unmodified 32-bit x86 Linux kernels as guest machines. Installation is as easy as running "modprobe lg" followed by "tools/lguest/lguest" to create a new guest.
Lguest can still be installed on kernel 4.14 and later through out-of-tree patches.
Lguest was maintained by Rusty Russell.