As far as being a hypervisor however, Vbox is strictly a type 2, i.e. it needs a host OS to run and resources are allocated to it.
In contrast, Xen and KVM are type 1 hypervisors, meaning they themselves allocate resources for guest OSes from bare metal.
There are debates about the semantics of whether KVM is type 1 or type 2 due to its implementation on top of the Linux kernel, but it is definitely in a different league than Vbox.
The problem is that the phrase used in Popek's paper is "Conventional OS". This was the 1970s. What's now considered a "Conventional OS" was not conventional back then.
For instance, everyone would agree Linux is a conventional OS but Linux provides a kernel module (KVM) that allows for virtualization driven by userspace.
As far as being a hypervisor however, Vbox is strictly a type 2, i.e. it needs a host OS to run and resources are allocated to it.
In contrast, Xen and KVM are type 1 hypervisors, meaning they themselves allocate resources for guest OSes from bare metal.
There are debates about the semantics of whether KVM is type 1 or type 2 due to its implementation on top of the Linux kernel, but it is definitely in a different league than Vbox.