When you have installed Xen on your CentOS 7 server using this guide :
https://www.virtualizor.com/docs/install/install-xen/
and on rebooting, the server hangs on a black screen with a blinking prompt then you will need to try to increase Dom-0 memory.
After the installation is completed, you can try to make an adjustment to the Domain0 memory parameters to. You’ll need to edit file /etc/default/grub
vi /etc/default/grub
The default for GRUB_CMDLINE_XEN_DEFAULT looks like this (For example):
GRUB_CMDLINE_XEN_DEFAULT="dom0_mem=1024M,max:1024M cpuinfo com1=115200,8n1 console=com1,tty loglvl=all guest_loglvl=all"
You can make adjustments to the memory so it can look like this :
GRUB_CMDLINE_XEN_DEFAULT="dom0_mem=2048M,max:4096M cpuinfo com1=115200,8n1 console=com1,tty loglvl=all guest_loglvl=all"
After you save your changes, run grub-bootxen.sh to update the grub2 configuration file.
/usr/bin/grub-bootxen.sh
After that command runs you can reboot your server. Once the server is back to a prompt, login and check running kernel :
uname -r
Next check Xen is running :
xl info
If it still fails to boot and shows a blank screen then you can disable I/OAT DMA Engine in BIOS. It was tested with the Dell server model R720 (gen. 12th) and server boots with Xen Kernel when I/OAT DMA Engine is off.