Installing 20+ operating systems on Mac without any virtualization
Depending on your Mac Pro the following OSs should run (or not). I didn't include every Mac Pro ever sold, but I tried to list major development steps ( e.g different EFI-architectures 32bit->64 bit):
MacPro1,1 MacPro3,1 MacPro5,1* MacPro6,1
1. FreeDOS +/- uc uc uc
2. Windows 95 - - - -
3. Windows XP ++ ++ ++ -
4. Windows 7 ++ ++ ++ -
5. Windows 8 - (++) ++ ++
6. Windows 10 - (++) (++) ++
7. Slackware + + + +
8. Debian + + + +
9. Ubuntu + + + +
10. Arch + + + +
11. Gentoo + + + +
12. Fedora + + + +
13. Xinu - - - -
14. Minix uc uc uc uc
15. FreeBSD +/- uc uc uc
16. NetBSD +/- +/- +/- uc
17. OpenSolaris + +/- +/- uc
18. OS X v10.0 - - - -
19. OS X v10.1 - - - -
20. OS X v10.2 - - - -
21. OS X v10.3 - - - -
22. OS X v10.4 ++ - - -
23. OS X v10.5 ++ ++ - -
24. OS X v10.6 ++ ++ ++ -
25. OS X v10.7 ++ ++ ++ -
26. OS X v10.8 - ++ ++ -
27. OS X v10.9 - ++ ++ ++
28. OS X v10.10 - ++ ++ ++
29. OS X v10.11 - ++ ++ ++
-: doesn't run ++: officially supported (++): runs probably +: runs maybe one/two drivers missing
+/-: some drivers missing uc: unclear *Newer MacPro5,1s probably can't run 10.6
I've compiled this list from various sources and it's probably not complete. I've embezzled the minor version numbers of the OS X versions (e.g MacPro1,1 needs at least 10.4.7). Some entries are well funded (like the Microsoft and Apple OSs), others are derived from my own experience or internet sources. A + or +/- doesn't necessarily mean that it is an easy job to get the system installed/booted. I welcome edits by more knowledgeable people though.
Addendum:
I've found two computer lookalikes which probably run all of those systems, the second one misses a video card though - and both are obviously no Mac Pros:
;-)
It cannot be done in hardware
Mac OS 10.0 through 10.3 only run on PowerPC machines. 10.4 and 10.5 could run on both (10.4 had separate versions for the two platforms, but 10.5 used a unified install). 10.6 and later can only run on Intel CPUs. Thus, there is no computer in existence that can run both 10.3 and 10.6. Also, 10.7 only supports 64-bit Intel CPUs (although a lot of its components include both 32- and 64-bit binaries), and 10.8 also requires 64-bit EFI firmware.
No Mac can run an OS older than itself - the hardware didn't exist to write the drivers for it.
Even in VM, no OS before 10.7 [maybe 10.5/10.6 server iirc, but not non-server] is licensed for VM.
Boot Camp for running Windows - no Mac old enough to run Windows XP can run Windows 10, because Boot Camp itself limits which OS can be installed.
For Unix variants, I invite edits…
Besides the issues others have brought up, with earlier Microsoft operating systems, you will run into the issue that they require MBR-formatted hard disks, which support a maximum of four partitions. You might be able to get around that with either multiple hard disks, or physically swapping out hard disks.
You may also run into issues with very old versions of Windows that may require hardware that your Mac no longer includes. I can't think of a specific example, though.