It's more secure due to automatic address space layout randomization. In fact, it's both space layout and space content randomization-- each application/kernel is unique.
in the case of Xen, you will have linux or *bsd in dom0 which will probably have aslr enabled. so, when you launch your unikernel and do aslr you will be randomizing an already random memory layout.
my point of contention is with the hypervisor+unikernel model. once you try to increase the tenant density of a particular piece of hardware with virtualization you lose most of the advantages of removing the os and re-introduce the complexity in a different way.