kvm is very high quality. When the SmartOS developers ported it to the OpenIndiana (or is it IllumOS? the naming is a joke) kernel, they found almost no bugs in kvm. Porting usually exposes bugs.
No it's not. It has its own set of problems. And bugs too! KVM has its pluses too, I agree, it's a VMM in kernel mode which apparently makes it faster, it is very non-intrusive to the rest of the kernel and hence got readily accepted upstream and so some might argue in that sense that it's better. But even though I have been hacking into KVM recently, for running a VM at home on my laptop/PC, I will always opt for VirtualBox for its simple ease of usage.
Can you give a few reasons from your own experience? I found it much, much easier to get Windows XP sound and USB passthrough working in virtualbox than in kvm.
Surely you're not saying KVM is bug free? Looks like it. That's almost certainly wrong.