Several different SIMH forks are included, along with a lot of other emulators; there are well over 150 different emulators, with some having multiple versions and variants present to handle things like regressions related to specific OSes.

Nested virtualization for certain x86 OSes running in QEMU is supported, although you will have to enable it manually (VirtualBox has a checkbox for this in its settings). For VMs that support it, the QEMU launch scripts will automatically use KVM if available and fall back to TCG if nested virtualization isn't enabled.