Starting with version 3.2, the Guest Additions of VirtualBox can change the amount of host memory that a VM uses while the machine is running. Because of how this is implemented, this feature is called "memory ballooning".
VirtualBox supports memory ballooning only on 64-bit hosts, and it is not supported on Mac OS X hosts.
To change on the fly:
#VBoxManage controlvm "VM name" guestmemoryballoon
where "VM name" is the name or UUID of the virtual machine in question and
To set a default balloon that will automatically be requested from the VM every time after it has started up:
#VBoxManage modifyvm "VM name" --guestmemoryballoon
Whereas memory ballooning simply reduces the amount of RAM that is available to a VM, Page Fusion works differently: it avoids memory duplication between several similar running VMs.
In a server environment running several similar VMs (e.g. with identical operating systems) on the same host, lots of memory pages are identical. VirtualBox's Page Fusion technology, introduced with VirtualBox 3.2, is a novel technique to efficiently identify these identical memory pages and share them between multiple VMs.
VirtualBox supports Page Fusion only on 64-bit hosts, and it is not supported on Mac OS X hosts. Page Fusion currently works only with Windows guests (2000 and later).
To enable Page Fusion for a VM, use the following command:
#VBoxManage modifyvm "VM name" --pagefusion on
Above content directly quote from virtualvox manual