I first assumed it was redirecting them to a library in user mode somehow, but actually the syscall is replaced with "int3", which also goes to the kernel. The whole reason why the "syscall" instruction was introduced in the first place was that it's faster than the old software interrupt mechanism which has to load segment descriptors.

So why not simply use KVM to intercept syscall (as well as int 80h), and then emulate its effect directly, instead of replacing the opcode with something else? Should be both faster and also less obviously detectable.