Is there a server version of VirtualBox available from Oracle for free?, not really. VirtualBox is a powerful desktop virtualization software that can be used on Desktop PCs and server hardware. It helps to virtualize the hardware resources on a physical computer and install several Operating Systems as virtual machine. How about server version?, the same VirtualBox can be installed on a physical server and run the virtual machines in headless mode as a service, so those virtual machines can be accessed remotely with network connection by RDP (Remote Desktop Protocol) or web interface, in this case, it can be called as VirtualBox server.
The latest Oracle VirtualBox 5.0 comes with further improvements to run virtual machines (VM) in headless mode as service and manage it properly. It’s like another free hypervisor for small size of IT infrastructure setup. By running a VM in headless mode as service, it consumes less resources rather than running on full GUI mode. VirtualBox server can be installed on any physical server OS including Linux and Windows and can host most of the leading Operating Systems as guest virtual machine. So if you want to download VirtualBox server, then download the normal version from official site here. ( Same version for Desktop)
Difference Between VirtualBox Server and Desktop Version
As described earlier, it depends on how you are going to run the virtual machines either by headless mode or full GUI mode. If you installed on a server hardware it can be called as VirtualBox server. There are few open source applications available to manage the VirtualBox services from web browser.
Here is the good scenario, the VirtualBox installed on a server, guest Operating systems are installed on NAS or iSCSI disks and all VMs started in headless mode where no output or indication of running VMs (virtual machines run like service in background without any GUI or outputs). Users will be accessing each virtual machines from their computer through remote desktop (RDP) software. Creating and managing virtual machines will be done through command prompt after connection established to the host server. These virtual machines can be moved to other hosts without downtime by VirtualBox teleportation which is called live migration.
When you use at home or office for personal use and install the Operating Systems on local disks, manage and view virtual machines physically in host computer through GUI (Graphical User Interface) as a single user , it can be called desktop version.
See Also ( Recommended readings)
As you aware the Oracle VirtualBox software should be installed top of an Operating System such as Windows, Linux and Mac OS X, not like other famous hypervisors like VMware vSphere and Microsoft Hyper-v which can be installed without Operating Systems.
Advantages of VirtualBox Server
1) Virtual machines will be centralized.
2) Less resource utilization on host because of headless mode.
3) Suitable for server Operating Systems where GUI is not important (core installation).
4) More secure than running GUI version.
5) Allow multiple users access over the network and physical presence is not required.
6) Teleportation/Live migration
1) Suitable for advanced users only for testing environment with more than one computer.
2) Lots of commands involved in command interface to configure and manage. If you are familiar with GUI, then you will see difficulty.
Oracle keep adding more features, enhancements and OS support in their every major version of VirtualBox. It’s a ideal virtualization software to install on a server for testing or even for a small production setup.