@jasonnix Unikraft is a framework to build a unikernel using usual application code, or with the elf loader, compatible regular linux programs.
Xen only allows you to start virtual machine where you still need some kind of operating system to do something relevant (like using network or storage).
Usually, we use a regular operating system on top of that (Linux distro, Windows, ...).
But what unikraft gives you a kernel, that "makes" a operating system with absolute bare essential (pv drivers, scheduler, memory allocator, ...) and you have the application fused in.
Without unikraft, you will have to basically implement a Xen pv kernel yourself.