QEMU is used by pretty much all open-source emulation or virtualization systems. Bits of it are even used by VirtualBox. QEMU on its own is not a virtualization system, just an emulator. KVM is the hypervisor most closely associated with QEMU, but it also works out of the box with Xen. Using QEMU with either of those hypervisors works just as well as VirtualBox for desktop and workstation virtualization needs, with the only significant downside being that QEMU isn't tightly coupled to a single standard GUI for management. Using QEMU for cross-architecture emulation (not virtualization) is slow, because that's an inherently high-overhead task. I don't think QEMU sets any records for being particularly fast or slow at cross-architecture emulation.
Running two x86 hardware-assisted hypervisors at once is madness. However, running one inside another is possible.
Running two x86 hardware-assisted hypervisors at once is madness. However, running one inside another is possible.