There is no "vSphere free offering", ESXi standalone is what you can run for free, with limitations.
Also, QEMU/KVM is not necessarily QEMU/KVM, KVM can be used separately, and so can QEMU.
And one last thing - in any article involving virtualization, it is important to explain the difference between a hypervisor and a full virtualization management product, as well as the many layers in between. vmkernel is not ESXi and is not vSphere, but people lump everything under VMWare and then do silly comparisons. A pure hypervisor is nothing more than a driver for the AMD-V/Intel VT-D CPU extensions, and nothing else. To turn that into a usable VM you need an emulator for the other hardware a VM has (which is where stuff like QEMU come in) with various levels of optimized hardware emulation and physical hardware access (paravirtualized hardware). These are already two layers of software just to be able to run a VM. And we left out the fact nothing can REALLY run on baremetal, metal needs drivers, so the "pure" hypervisor is really one of the drivers that exist in a set of drivers, schedulers and supporting software, aka the kernel. Xen is one such kernel with the hypervisor included. Linux with KVM makes another such kernel. On top of that you have the base management layer, so that you don't need to type in a 15-line-long command just to get a VM going, this is where you have stuff like libvirt, ESXi and so on. And the you get the datacenter level management layer (vSphere, oVirt) or the IaaS management layer (Openstack Nova, EC2 etc)