For now pure PXE is not supported in a Ravello vm. iPXE can be used as an alternative.
To achieve it, follow next steps:
1) setup DHCP and TFTP servers in the same application (as one vm or two different vms). These servers must have a static IP configured inside them, and configured as such in ravello mgmt.
2) Configure the DHCP server to allocate IPs based on MACs (DHCP Reservation)
3) Get an iPXE ISO (from http://ipxe.org/).
4) Add it as CD to the designated vms (pxe clients) and boot from it.
5) In Ravello MGMT set guest IPs as "Static IP" that correspond to the DHCP configuration from the previous step.
*! All vms MUST be set with static IPs only and all in the same subnet (same L2 network).