Please add the following entries in order to boot the xen kernel

In /etc/sysctl.conf:
	vm.max_wired=-1

In /etc/ttys:
	xc0 	"/usr/libexec/getty Pc"         xterm   on  secure

In /boot/loader.conf for a dom0 with 2G memory and 4 vcpus:
	hw.pci.mcfg=0
	xen_kernel="/boot/xen"
	xen_cmdline="dom0_mem=2048M dom0_max_vcpus=4 dom0pvh=1 com1=115200,8n1 guest_loglvl=all loglvl=all"

Add to the above xen_cmdline in order to activate the serial console:
	console=com1

In /boot/menu.rc.local:
	try-include /boot/xen.4th