Typically the use of the PMOS and NMOS both (Complimentary Gm) can be the most efficient power, but up to a point.  The limit is that the tank has a smaller voltage limit due to a dual constraint of PMOS in the up direction and NMOS in the down direction, with  that the best phase noise is more limited than say a PMOS or NMOS only.  When a NMOS or PMOS only design is made the Tank is usually connected to the supply or ground thus the VCO amplitude on the Tank can be twice or more of the since it can swing above supply or below ground in the PMOS case.  But as a result the dBcPnoise/IC efficiency.  

Note using a tank to ground as in the PMOS gm case can cause the parasitic diodes to turn on momontarrily and this can cause some problems, which may not be modelled properly.

thanks for your replies.

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i think the above reply is refering to pmos/nmos only LC tank right?
How about CMOS LC tank case? does it need current source too?

hi casual
           i think with out current source you will get very good phase noise due no 2*fo up conversion as well as more swing. Coming to supply noise if you put good supply regulator probably you would end up with less phase noise with less power.

Thanks,
Rajasekhar.