High NTP offset and jitter while delay is low. This can be seen with "ntpq -p".

Offset is the time difference between the local server and remote
Jitter is the difference between the last and current offset measurements, thus if it is high, it means that the offset is increasing more over time.
Delay is the time that it takes to communicate with the remote server. A low delay means that the issue is not related to network delays.

This measurements tell that NTP is not being able to discipline the clock as it drifts faster than it is able to sync.

This is a known issue and a workaround is available

1) change clock source to xen after reboots:
echo 'xen' > /sys/devices/system/clocksource/clocksource0/current_clocksource

2) Make change persistent after reboots:
echo "echo 'xen' > /sys/devices/system/clocksource/clocksource0/current_clocksource" >> /etc/rc.local

3) make boot scripts executable: 
chmod +x /etc/rc.d/rc.local

4) Leave it running for a couple days and monitor offset and jitter with ntpq -p

This is going to change the clock source to xen instead of tsc.

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Problem Cause

Under investigation