Can a system run multiple virtualization technologies simultaneously?

Can one system, for example, run both Red Hat Virtualization and VMware virtual machines at the same time?

No.

Virtualization software needs exclusive control over certain hardware resources. The host system may crash or reboot if multiple virtualization systems are active and try to use the same hardware resources. Be sure to install and use only one virtualization technology on a system.

To check if Red Hat Virtualization (Xen in Red Hat Enterprise Linux 5) is active, look for "xen" in the uname output, for example:

$ uname -r | grep xen
2.6.18-8.el5xen

To disable Xen, disable the xend and xendomains services, then reboot and choose another kernel at the GRUB screen, or use grubby to change the default kernel and reboot. For example:

# chkconfig xend off
# chkconfig xendomains off
# grubby --set-default=/boot/vmlinuz-2.6.18-8.el5
# reboot