How to select paravirtualization interface in VirtualBox?

The VirtualBox Manual, in the section titled Paravirtualization providers explains very clearly when each should be used (emphasis added):

• Minimal: Announces the presence of a virtualized environment. Additionally, reports the TSC and APIC frequency to the guest operating system. This provider is mandatory for running any Mac OS X guests.

• KVM: Presents a Linux KVM hypervisor interface which is recognized by Linux kernels starting with version 2.6.25. VirtualBox's implementation currently supports paravirtualized clocks and SMP spinlocks. This provider is recommended for Linux guests.

• Hyper-V: Presents a Microsoft Hyper-V hypervisor interface which is recognized by Windows 7 and newer operating systems. VirtualBox's implementation currently supports paravirtualized clocks, APIC frequency reporting, guest debugging, guest crash reporting and relaxed timer checks. This provider is recommended for Windows guests.

The other options in the drop down do the following, as explained in the vboxmanage docs:

• None: Specifying none explicitly turns off exposing any paravirtualization interface.

• Default: The option default, will pick an appropriate interface depending on the guest OS type while starting the VM. This is the default option chosen while creating new VMs.

• Legacy: The legacy option is chosen for VMs which were created with older VirtualBox versions and will pick a paravirtualization interface while starting the VM with VirtualBox 5.0 and newer.

Let's be more precise, because I had 4.3 migrated VMs, and those are "legacy", also I was interested in what setting to use, if I copy VM from Linux to Windows or vice versa:

--paravirtprovider none|default|legacy|minimal|hyperv|kvm: This setting specifies which paravirtualization interface to provide to the guest operating system.

• Specifying none explicitly turns off exposing any paravirtualization interface.
• The option default, will pick an appropriate interface depending on the guest OS type while starting the VM. This is the default option chosen while creating new VMs.
• The legacy option is chosen for VMs which were created with older VirtualBox versions and will pick a paravirtualization interface while starting the VM with VirtualBox 5.0 and newer.
• The minimal provider is mandatory for Mac OS X guests,
• kvm - recommended for Linux guests
• and hyperv - recommended for Windows guests

These options are explained in detail under Section 10.4, “Paravirtualization providers”.

From manual chapter 8.

So if you have upgraded VirtualBox to version 5 in Linux, just select kvm or default, start VM and check if it works correctly. If yes, just leave new setting.

If you migrating/moving VMs from Linux to Windows, I recommend to have default option, which makes automatic selection of best virtualization interface.