By default, VirtualBox creates virtual disk images in a special format called VDI, which is unique to VirtualBox. Disk images are stored in $HOME/.VirtualBox/HardDisks.
You’ll need to convert VDI into another format if you want to run a VirtualBox VM on another virtualization platform, such as VMWare or KVM.
The VMDK virtual disk format is a good choice because even though it originated with VMWare it is supported by other virtualization platforms including VirtualBox and KVM.
VirtualBox enables the conversion using the low-level “VBoxManage clonehd” command:
VBoxManage list hdds # prints a list of disk image UUIDs
ls -la converted.vmdk
Once you have converted to VMDK you can use qemu-img, a tool bundled with qemu (KVM’s virtualization backend) to further convert VMDK to other formats.
A particularly useful format to convert to is ‘raw’ which you can then mount as a loopback device:
apt-get install qemu
qemu-img convert -f vmdk converted.vmdk -O raw converted.raw
mount -o loop converted.raw /mnt