For example, if I want to test a ARM platform I use qemu because it is a processor emulator. I use VirtualBox for ISOs because that works for me.
virt-manager hands down, which primarily uses qemu-kvm underneath. If I am doing ARM, its generally in a chroot with qemu-user-static, and not in a virtual machine, cause that is alot easier to work with.
kvm is the only virtualization platform which doesn’t have driver issues. Also: it supports NESTED virtualization on AMD and Intel chips, which is pretty insane.