High CPU load but top processes in top all around 0%

I think this bug is your case. From what I see from the output, you have enough memory (note the cached 14 GB or so), no I/O issues, but you have xen-related processes running. This make me think it is a bug.

Load is a measure of the workload a system has had on a 1, 5 and 15 minute basis.

The most common misconception is that Load Average is purely connected to the CPU usage of a system.
Load does however incorporate additional measurements such as CPU waiting for I/O which I think is your issue.

Based on the image I'm guessing you ran out of memory and started swapping data to disk.

A simple free -m will tell you how much RAM and swap is used.
The interesting column is the free column besides -/+ buffers/cache.
If it's close to zero you've run out of RAM and should act accordingly.

Noticed that the load average is quite high (68, wow). Is it possible that there are a lot of processes which takes up a little bit of CPU, thus add up consuming all CPU time? Maybe, those processes just start and finish very quickly thus top cannot capture the existence of them, you may try to see if atop can see that or not.

Try

iotop

IO was it for me most of the time.