With qemu you are running everything in usermode (with kqemu, you offload some to the kernel, and with kvm/qemu you use processor extensions to allow non-trusted code ring code pseudo-access to ring 0 -- this allows virtual machines to run at basically native speed, hence why kvm is so fast), so there is plenty of overhead, but it is quite secure since.
Originally Posted by kebabbert
VMWare offers full emulation as well, but it also allows paravirtualization (so it is fast but requires changes to the guest OS).
Containers are completely different. They are really only useful if you are running a single OS type (though I believe Solaris, at one point, allowed linux containers inside of solaris, but I don't know if that is still the case, nor what kind of overhead was required), but if you are running an OS server to lots to terminals, containers should be great, though there might be even better solutions (LTSP).
Regardless, I think we can say that Oracle calling Solaris the only true cloud os is a bit misleading at best, if they are basing this off of its ability to virtualise various parts of itself.