... like running a PPC or ARM system on x86 hardware.
As far as I know:
- VMWare can't
- QEMU can
- KVM can because it uses QEMU
Is this correct? What other solutions are there?
That's not virtualisation, that's emulation. Virtualisation can be seen as a specific kind of emulation where you emulate the same machine on itself, accelerating the emulation by letting some of the instructions run for real.
Your question should just be "What emulation software exists?", which is probably too wide a question to be useful. Wikipedia lists quite a lot.
Virtualisation software (hypervisors) won't do this at all. VMWare is a hypervisor and QEMU is an emulator - they are fundamentally different technologies.
Depending on what you are trying to emulate a suitable emulator may or may not be available. For example, there is an IBM mainframe emulator called 'Hercules' that will let you boot up mainframe operating systems, and some older versions of these systems can be downloaded for free. Other emulators may really only be suitable for debugging or system development work, as opposed to running legacy applications.
PearPC is a Power PC emulator for x86 platforms (Windows and *IX I think)... not sure if you can run AIX on it yet but I've heard plans on it - something I'd really want to do myself for testing purposes - but at least it runs some of the more common PPC operating systems ^^