Linux-KVM, QEMU, Virtualbox, VMWare

It depends. I like Virtualbox because of its GUI—it's simple but complete—and speed. But I also use kvm-qemu with libvirt, because it's faster and more configurable than Virtualbox. I don't like VMWare (Workstation) because it's commercial, closed source and really heavy. I like it only because it was one the first commercial companies which supported Linux by releasing VMWare Workstation and Server in the 90's. For virtualization there are a lot of products under Linux, you choose the product you like.

X86: VMWare, qemu, kvm, bochs (slow but works well with older operating systems), pc emulator
PPC: PearPc: very slow, claims to emulate old ppc mac-os
Sparc: qemu, slow and with cpu at 100%, emulates old solaris version
Alpha: some commercial emulators, good speed
Itanium: none

There are also a lot of emulators for unmaintained and old platforms, like basilisk, sheepsaver and a lot of others.

Are there more popular virtualization systems than the ones I mentioned above?

You listed almost all popular virtualization systems, except 'Xen'.

When to use what?

Since you are using Ubuntu box, I suggest qemu/kvm for you. You can start with 'virt-manager', which is 'GUI front' of libvirt/qemu/kvm, and looks very similar to 'vBox or VmWare Workstation' on windows.