In VirtualBox / KVM:
* `ctrl + c` in a VM leads to copying the contents into the VMs clipboard as well as into the host clipboard as well as into the clipboard of any other VM
* `ctrl + c` on the host leads to copying the contents into the host's clipboard as well as into the clipboard of any VM
* this is non-ideal for security since in many cases VMs are used to compartmentalize things.
* `ctrl + c` / `ctrl + v` takes effect only inside the VM.
* Each VM has its own independent clipboard.
* The Qubes host (dom0) has its own independent clipboard.
* Qubes introduced the concept of a global clipboard.
* To global copy the user has to run `ctrl + shift + c`. This copies contents into the global clipboard.
* A subsequent `ctrl + shift + v` pastes it into one other VM.
* ("global clipboard" is a non-ideal name. The "global" clipboard cannot be read by other VMs in which `ctrl + shift + v` was not used.)
* After `ctrl + shift + v` the global clipboard gets cleared to prevent accidental leakage into another VM.
* These are my words. This is how Qubes describes the feature: 
rehash and post feature requests against #VirtualBox and #KVM