Selected answer is incomplete and not completely correct.
PXE clients do not have hardwired any IP.
When they boot up they broadcast a DHCP request identifying themselves as PXE clients.
PXE mainly uses 2 protocols DHCP and TFTP. Basic DHCP services provide to the booting station IP/MASK/Gateway while DHCP answers for PXE identified clients include 2 additional parameters: TFTP server IP and file name of the NBP (network bootstrap program) that is the first network transferred piece of code taking control when a PXE client boots up.
TFTP is rather a spartan protocol UDP based (slow, not reliable transfers), after the first NBP takes control it usually installs a full TCP stack and the bulk of the booting/installation components can be then transferred relying on a faster/more reliable transfer protocol like HTTP, FTP, NFS, CIFS, its all depends an what you are PXE booting/installing.