I would say that not KVM uses QEMU to emulate hardware, but QEMU can use KVM to emulate processor and memory.
So, if you use KVM, you almost have no overhead for CPU and memory, but have overhead on network and disc IO, that are still emulated in QEMU. Virtio (which is supported by most Linux distributions and even Windows) can make this overhead very low.
Comparing to OpenVZ, QEMU+KVM gives you a lot of benefits: you can use custom kernel and non-Linux OSes, VPSes are better isolated.
QEMU without KVM is almost unusable for VPS hosting, because it slows down machine CPU nearly 50-200 times.