Normal mode is when iPhone was started in the usual way, going through a secure boot chain. The boot ROM contains the apple root CA public key is used to verify the next stage low level boot loader (LLB) is signed by apple or not before executing it. LLB finishes its work and load the second stage boot loader, iBoot, which load the iOS kernel
Recovery mode is entered if any stage in the secure boot chain fails. The iPhone requires an upgrade or restoration to exit from recovery mode.
Device Firmware Upgrade (DFU) is entered when boot ROM cannot verifies or loads LLB. In DFU, a different boot sequence will be used. Boot ROM runs iBSS, a modified version of iBoot. It in turn runs iBEC that loads the kernel which loads the RAM disk.
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