Different approaches to isolation. A kata container is using Clear Linux to load a feature-complete Linux kernel into tiny VMs (disclaimer: I do not know exactly how it's different from any other VM,) a unikernel is a small bare-metal "library" that gives you minimal OS-like functions to put in a hypervisor to run your application. Unikernels are still more minimal, I'd guess.

Here's a recent paper about the unikernel approach

http://cnp.neclab.eu/projects/lightvm/lightvm.pdf