I am using Ubuntu 18.04 OS.

OnDemand Governor is not found in /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors .

It shows only i) powersave and ii) performance

that is what I got, and when it is on powersave, is just runs slower until needed to be faster.. so yes it is basically on demand. if it is on performance it runs faster, and if it need to be faster then it goes to the turbo speed max GHz as needed. so it is still on demand. it is just how fast do you want your CPUS to "idle", basically.

May I know what is difference between power save and that ondemand and conservative governor?
Does Powersave plan has only 3 stage : slower, faster , and turbo ?

Probably because you are using Intel_pstate and not acpi-cpufreq. Check with command cpupower frequency-info of cpufreq-info to know which model you are using. Add intel_pstate=disable to your boot parameters to use acpi-cpufreq. Personally I like acpi-cpufreq alot more than Intel Pstate. I think odemand and conservative are by far the best governors available.. In particular my favourite is Conservative.

Powersave is a hyperbole and a joke, and "absolute opposite" of performance. I don't like it, the performance is poor and it scales badly. Governor tests done by professionals backs this claim up. Powersave is BAD.

Conservative is awesome in my book, very similar to ondemand, but more conservative. It's exactly what a CPU governor should do in my book. Ondemand is pretty good too, but conservative is absolutey awesome.

For me:
If scaling
then acpi_cpufreq
&& set -g conservative

Always, always, always.

PS. Ondemand performance is slightly better than conservative, but I prefer to use less power (and increase battery time) and increase the cpu lifespan.

Except it does not work well, and turbo speed is not clever if you want to maintain your CPU for a long time. This used to and probably still is the issue with Windows, it has a tendency to overheat the CPU and shorten the lifespan. Intel_pstate very much remind me of the poor policies of Windows.

Always max or always minimum, seems to the only two options.

acpi_cpufreq is absolutely a wonderful work and ondemand and conservative are great, conservative is perfect. Unless ofcourse you work with very performance dependent software and always need to max your CPU and don't care about the computer longevity.

It's generally always on lowest, while ondemand and conservative scales up better when you need more performance.