Software-based

Software-based virtualisation adopts a process called deprivileging, for which guest's supervisor instructions are translated into host user space. The VMM further intercepts all guest modifications to kernel data structures, such as the IDT (Interrupt Descriptor Table), redirecting them to internally managed replicates, when necessary. Another example of these "shadow tables" are page tables: the hypervisor catches the guest OS when attempting to set its own page table, and it builds a corresponding shadow table where guest "physical" pages are translated to host virtual ones.

Problems

Some architectures originally couldn't be efficiently virtualised because not all "sensitive" instructions were privileged.

A guest supervisor could detect deprivileging by reading some hardware registers seeing mismatched CPL (Current Privilege Level) or host tables, exposing the VMM presence. Furthermore, constant emulation (sysenter/sysexit called for every guest syscall) required too many traps, making software-only VMMs impractical.