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Hi, I am interested in virtualization and kernel debugging. On github I can find simple kernels, for example hach-que and jtsiomb, but I need also simple virtualizers, much more smaller than QEMU or VirtualBox.

if it doesn't have to be open source check out armv8 foundation model, its a very crude implementation of a 64 bit ARM board without PCI, USB or peripherals besides a screen and a timer, it has a small controller that controls all devices and you can figure out how to use it in under 5 minutes. It's also debuggable by gdb or other tools

mupen64, pcspim (super small), 6502 emulators are also good options.

If you have the x86 PC in mind, smaller and simpler is a problem. The x86 PC itself is not small/simple. Especially the modern one. If, OTOH, you consider an older x86 PC, e.g. 80186-80386-based, it's much easier to grasp the whole thing and emulate/virtualize it. Or you could consider a different architecture / device.

Also, since this is very resource intensive and non-trivial, simplistic solutions tend to be pretty slow and very limited/buggy, meaning it will likely be hard to find something usable and at the same time small/simple.