Preboot Execution Environment (PXE) is a special boot type that downloads an image from a server on the network. Fixed Drive is a boot type that uses an internal HDD or SSD. eSata is a type of external drive attachment. FTP is a protocol for transferring files and not a boot type.
In computing, the Preboot eXecution Environment, PXE (most often pronounced as pixie) specification describes a standardized client–server environment that boots a software assembly, retrieved from a network, on PXE-enabled client (NIC), and uses a small set of Extensible Firmware Interface industry-standard network protocols such as DHCP and TFTP The concept behind the PXE originated in the early days of protocols like BOOTP/DHCP/TFTP, and as of 2015 it forms part of the Unified Extensible Firmware Interface (UEFI) standard In modern data centers, PXE is the most frequent choice for operating system booting, installation and deployment