Well, as long as you have some sort of decent virtual machine software installed on it, OVF is supposed to be the common package for cross portable VMs - you can run or import an OVF on virtualbox and vmware.
Considering your requirements virtualbox might be the best option - runs on the big 3 oses and has a portable version on windows (which requires admin rights). It exports to OVF and can read them (and i recommend keeping the VM in that format, rather than the native one, for better cross host portability, in case you needed to run it on VMware hosts)
If you want it to boot off a USB drive, you'd probably be able to run virtualbox off a USB stick. Having it totally portable is tricky - you could use a USB hard drive, and have a linux partition (for a bootable OS to run the VM host on),and have portable VM software (eg virtualbox portable) and the VM on a second drive (NTFS will likely be best for this, as long as your OS X systems have macfuse, else go FAT32, and have split volumes for your VM).
Another alternative to look at, though much worse performance-wise is QEMU. It runs networking WITHOUT an install or admin rights, and once again runs on the big 3. No idea of OVF support - it has its own format.