Qemu is a whole-system or CPU emulator, originally for x86 PCs but now encompassing other systems and architectures.
Where the virtualization bit comes in is with kvm. Kvm is a Linux kernel module that lets you take advantage of x86 CPU hardware virtualization. Qemu can use kvm to provide the virtual CPU while it itself emulates the rest of the PC hardware, resulting in a complete virtual system.
That said, qemu can also emulate the CPU itself, allowing for example an x86 OS to run on an ARM system, or vice versa, but that of course is slower.
My Workstation runs on Windows, and up until some years ago I was using VMware Workstation for virtualization, and then moved over to VirtualBox, which I've also started to use on a Windows-based home server. I've always used both of them via the GUI.
Now, if I want to build a beefy Linux server (headless) and run some virtualization on it, would I then use QEMU and get the same thing I've been getting with VirtualBox on Windows?
How do both of these compare performance- and feature-wise? Are they alternatives, or do they server different purposes?