Mac Pro 3.1 with Yosemite won't boot after GenericUSBXHCI USB 3.0 .kext added I was trying to install drivers for my NEC USB 3.0 PCI card.
( propably http://sourceforge.net/projects/genericusbxhci/files/
or https://github.com/RehabMan/OS-X-Generic-USB3 )

I rebooted my mac into 
    kext-dev-mode=1

then i typed:
    sudo kextcache -m /System/Library/Caches/com.apple.kext.caches/Startup/Extensions.mkext /System/Library/Extensions

GenericUSBXHCI .kext was enabled. Two other .kext too.

After restart - my usb 2.0 and usb 3.0 was working fine! Pendrvive was copying files very fast.

But unfortunetally magic mouse won't connect, so i restarted Mac Pro.

Now, booting is frozen. Sometimes at 1/3, sometimes at 1/2
I can't enter the Recovery Mode or Single user mode.

I was trying every possible ctrl+r, windows+r, alt+r, shift combinations on my logitech usb keyboard, but Yosemite just won't boot.

After freeze, computer reboots, and white screen of death appears.
Over and over again.

I have Yosemite installation disk image on my pen drive and Mavericks installator on my external hdd, but i wasn't able to start it.

I don't event know if efi sees my keyboard.

Maybe keybindings are the problem, cause i normally use oldshool DIN-5 mechanical keyboard from 1980' But the shortcuts from this keyboard won't work too.

What can I do to enter command line or repair .kext's ?

I have no other mac/pc to connect the hard drive.

UPDATE:
First success: Linux Mint booted and it's working fine. I can read my Mac SSD drive!
So i assume that hardware is all right. Even magic mouse is working under linux.

I have discovered installer.faliurerequests file on Mac drive. It says:
Operation fsck target CrashAtProgress 0

After rebooting from linux to Os X it was working like 10 minutes, artifacts appeared on the screen, and then crash again.
Next time it was working only 2 minutes... and now i can't boot it again.
The problem was graphic card! GeForce 8800 GTS was making all that problems.
I have replaced it with Radeon 5770, and everything is fine!

Funny thing is, the USB 3.0 really has something to do with wireless magic mouse! USB 3.0 somehow interferes with bluetooth 2.4GHz waves!
It can be repaired with... aluminium foil!
I've covered bt module with aluminium foil, and mouse is working fine again!

Everything solved