Your KVM and VirtualBox images should be on same partitions,
in case you use several HDDs or filesystems.
1. download linux-2.6.18.tar.bz2
2. # tar xvf linux-2.6.18
3. # make defconfig
-- Start timer (I use handwatch)
4. # make
-- stop timer
Host: Mandriva Linux 2010, x64, Core 2 Q6600, VBox 3.2.0
Guest: RHEL 5, x64, 1 CPU, SATA + Host I/O cache
kernel compile time: 12:20
Guest: RHEL 5, x64, 1 CPU, IDE + Host I/O cache
kernel compile time: 12:15
In other words, I got the same speed from IDE and SATA.
-----------------
Qemu/KVM
Host: Mandriva Linux 2010, x64, Core 2 Q6600, Qemu/KVM v0.11.0 (default with Mandriva 2010)
Guest: RHEL 5, x64, 1 CPU, SCSI
Guest OS Install failure. (Qemu bug?):
lsi_scsi: error: Unimplemented message 0x0c
Guest: RHEL 5, x64, 1 CPU, VirtIO-blk
kernel compile time: 10:20
Guest: RHEL 5, x64, 1 CPU, IDE
kernel compile time: 8:56
-Technologov