I've tried to run Windows XP guest using VirtualBox in Ubuntu 10.04 (Lucid Lynx) and it was running significantly slower than qemu/kvm. But simple tricks made it run much faster.

The worst slow down was caused by I/O APIC. It is used when you want your Windows XP guest to use dual or more core. I didn't make extensive tests, but the following Settings

• IO APIC Disabled
• 192MB RAM
• One CPU core for guest Windows XP
• VT-x/AMD-V Disabled
• Nested Paging Disabled
• PAE/NX Disabled
• EFI Disabled
• 2D/3D Video Acceleration Disabled

gives much much better performance than

• IO APIC Enabled
• 784MB RAM
• Dual CPU core for guest Windows XP
• VT-x/AMD-V Enabled
• Nested Paging Enabled
• PAE/NX Enabled
• EFI Disabled
• 2D/3D Video Acceleration Enabled