iLO drivers enable to take advantage of features of iLO management engine in HP Proliant servers. iLO drivers are targetted for HP Proliant Gen 8 systems and above which have iLO 4 management engine. 
Currently there are 3 iLO drivers:
The iscsi_ilo and agent_ilo drivers provide security enhanced PXE-less deployment by using iLO virtual media to boot up the baremetal node. These drivers send management info through management channel and separates it from data channel which is used for deployment. iscsi_ilo driver uses deployment ramdisk built from diskimage-builder, deploys from Ironic conductor node and always does net-boot. agent_ilo driver uses deployment ramdisk built from IPA, deploys from baremetal node and always does local boot.
pxe_ilo driver uses PXE/iSCSI for deployment (just like normal PXE driver), but support automatic setting of requested boot mode from nova. This driver doesn't require iLO Advanced license.