What are the differences between QEMU and VirtualBox?

Basically both have features which the other does not have, so this might ease the decision. QEMU/KVM is better integrated in Linux, has a smaller footprint and should therefore be faster.

VirtualBox is a virtualization software limited to x86 and amd64 architecture. Xen uses QEMU for the hardware assisted virtualization, but can also paravirtualize guests without hardware virtualisation. QEMU supports a wide range of hardware and can make use of the KVM when running a target architecture which is the same as the host architecture.

Xen is a Type-1 hypervisor where VirtualBox and QEMU are considered as Type-2 hypervisors (also there might be a debate considering kvm being a kernel module).

A similar question has been asked before in this community.

QEMU with KVM is much, much faster than VirtualBox, you can test it yourself:

VirtualBox:

QEMU

Disk and CPU tests provided similar results, more or less.

A difference is the supported list of instructions. Virtualbox and VMware don't support the f16c-instructions supported by architectures beginning with Ivy Bridge, which limits compilations even with newer CPUs to those for Sandy Bridge and leads to other incompatibilities.