This is a known bug in Red Hat. Please follow the belw link to reach out the bug details
Bug 645940 – Kernel will not boot, just sits at ‘starting udev’
Clocksource is the name of the new time keeping subsystem introduced in linux 2.6 kernel. In the subsystem there are several clock sources that can be used to read the time. One of them is the tsc clocksource which uses the time stamp counter that is present on each processor and can be used to track how much time has elapsed. The TSCs on the different processor are not necessarily perfectly in sync with each other so time can appear to go backward if the TSC is read on one processor and then another processor. Early version of the TScs clock source did not handle this case, which can be lead to the guest operating system not responding. Due to the different timing behavior of running in a virtual machine this can be observed more frequently.
Append clocksource=acpi_pm in your grub.conf file with the kernel entry as shown in below screenshots
Follow the screenshots
Press the ‘e‘ button to goto the edit option
Go to the kernel option and again press ‘e‘ and add the following at the end of the line
Press enter to save and hit ‘b‘ to boot your machine.
Make sure to add this entry in your grub.conf file to avoid future issues in the machine
# vi /etc/grub.conf
title CentOS Linux Server (2.6.18-238.el5PAE)
kernel /vmlinuz-2.6.18-238.el5PAE ro root=LABEL=/ rhgb quiet clocksource=acpi_pm