[wmf]

VMs usually do have BIOS (sometimes you can see it flicker on the screen) but like NeutronBoy said, the hypervisor just creates the virtual hardware devices in a pre-initialized state so the BIOS has to do almost no work and it completes in a fraction of a second. Clear Containers boots even faster by not using BIOS; the hypervisor directly loads the kernel and initrd into RAM. So in this case "booting" means starting the kernel, mounting the root filesystem (accelerated using DAX), running init, starting dockerd, etc.

[rwmj]

Actually Clear Containers does have a BIOS, because Linux requires one eg to read E820 data and to set up the virtual video. However the one Clear Containers uses (from kvmtool) is extremely minimal -- it's literally enough of a BIOS just to answer the int calls that modern Linux makes at boot and that's all. IIRC it's hundreds of lines of code only.