They're the same chip essentially. Assuming you're talking about desktop chips here...
i7's (non "K")
- quad core with hyperthreading
- has integrated graphics
- no overclocking
i7 "K" Processors
- as above, but allows for overclocking on the right motherboard and with the right cooler
Xeon E3 processors
- as above, but no overclocking
- most do NOT have integrated graphics - you'd need a PCIe graphics card
- support ECC RAM (which you probably don't need, but is used in servers for maximum reliability)