VirtualBox KVM uses the same format as VirtualBox proper.

The virtual machine description is not compatible with Proxmox. That said, you can import a VirtualBox environment into Proxmox and vice versa. I have done it.

What you actually import is the “drive” of the VM. You create a new VM in Proxmox (or VirtualBox if going the other way). Then you attach the imported disk to it.

The devices may differ between environments. So they are not exactly the same. But as long as the OS you are visualizing can support both, it works well.