Use virtIO for disk and network for best performance.
Linux has the drivers built in since Linux 2.6.24 as experimental, and since Linux 3.8 as stable
FreeBSD has the drivers built in since 9.0
Windows requires the Windows VirtIO Drivers to be downloaded and installed manually
The IDE option works out of the box, VirtIO and VirtIO SCSI do not without having the guest drivers attached on another disk drive and manually installing them. This is hard for firsttime virtualizers, so it is not the default.