I was on a call with Starwind this morning including a sales person and a techie. KVM wasn't mentioned as an option. Perhaps that's because we were discussing the Hyper Converged Appliance and KVM isn't currently an option there. I'm honestly not sure though, I'd have expected them to mention it if it was going to be their focus in the future.
I was looking at the appliance because the price I was quoted for just the VSAN product i.e. the software without any of the hardware, was almost as much as the appliance (and for less usable storage space).
I know Starwind do the free version but since it lacks the management application I was sceptical about using this due to the fact I'd need less technical people to support it in my absence.
Agent versus agentless backups isn't a driving factor for me. I mentioned it because it's something I currently have. I'm not ideologically wedded to it.