All of them are small and lightweight, then various layers of crap have to be added for them to work such as drivers and machine management. Lightweight is marketing directly aimed at OCD techies who like things to be optimised and small.
Xen also had the unfortunate problem of being used by Citrix which effectively killed it and caused the birth of KVM, which has the unfortunate problem of not being VMware or Hyper-V and lets face it Hyper-V only made progress because it's bundled with Windows and is cheaper than VMware to buy.
In reality, Nutanix don't need their own, they just need to package a hypervisor so that the install is slick with a single package. Xen or KVM will achieve that as long as they play nicely with the licence folks (yup, looking at you VMware...)