Network adapters in Virtualbox virtual machines
The most important difference between these adapters is that only some of them might be supported by a particular guest system — so this is a difference between “works” and “does not work at all”. E.g., Windows Vista and later versions no longer have a driver for AMD PCnet cards, therefore one of Intel PRO/1000 variants must be used for these systems; however, older systems might have a driver for PCnet, but not for E1000. Three different variations of Intel PRO/1000 cards are supported for the same reason.
See the “Virtual Networking Hardware” section in the VirtualBox user manual:
The PCNet FAST III is the default because it is supported by nearly all operating systems out of the box, as well as the GNU GRUB boot manager. As an exception, the Intel PRO/1000 family adapters are chosen for some guest operating system types that no longer ship with drivers for the PCNet card, such as Windows Vista.
The Intel PRO/1000 MT Desktop type works with Windows Vista and later versions. The T Server variant of the Intel PRO/1000 card is recognized by Windows XP guests without additional driver installation. The MT Server variant facilitates OVF imports from other platforms.
If you create a new virtual machine and select the OS type correctly, an adapter type which is compatible with the selected OS will be chosen automatically.
However, there are other differences between adapter types, which should be considered when choosing between multiple types which are all supported by the guest OS:
Some advanced features such as jumbo frames and 802.1q VLAN tags are supported only when using one of E1000 adapter types.
If you need to boot guests from the network, the open source version of VirtualBox provides the Etherboot-based PXE ROM only for PCnet adapters. The Intel PXE ROM for E1000 is available only in the proprietary Oracle Extension Pack. However, you can supply an external file with a PXE ROM, and all of those adapter types (including virtio-net) are supported in gPXE, so this is not a very important restriction.
E1000 variants usually provide slightly better performance than PCnet. The virtio-net adapter in theory should provide the best performance, because its “hardware” interface was created with virtualization in mind; however, some time ago its support in VirtualBox was buggy, and I did not try it recently.