Difference between m_sequencer and p_sequencer in UVM

To understand difference between m_sequencer and p_sequencer, let's first go through couple of classes from UVM library.
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When you call `uvm_declare_p_sequencer from sequence, it overrides m_set_p_sequencer function.
When you call start method of sequence, it calls set_item_context method,
Then set_item_context method calls set_sequencer method,
Then set_sequencer method assign handle of sequencer (which you pass in start method of sequence) to m_sequencer,
set_sequencer also calls m_set_p_sequencer method, but you have override the m_set_p_sequencer method by declaring `uvm_declare_p_sequencer macro.
So m_set_p_sequencer defined in macro will be executed, and it casts m_sequencer into p_sequencer,
where p_sequencer's type is specified by the argument of `uvm_declare_p_sequencer macro.

Conclusion:

m_sequencer and p_sequencer both point to the same thing (the sequencer on which the sequence is running). However, there are some important differences:
m_sequencer is a generic uvm sequencer pointer of type uvm_sequencer_base. It will always exist for an uvm_sequence and is initialized when the sequence is started.

p_sequencer is a reference to uvm_sequencer#(REQ,RSP), the user derived parameterized sequencer class.
Other way you can say that p_sequencer is a type specific sequencer pointer.
p_sequencer will not exist if the `uvm_declare_p_sequencer macros aren't used.

Advantage of p_sequencer over m_sequencer:
Being type specific, you will be able to access anything added to the sequencer (i.e. pointers to other sequencers, handle of environment class (which is defined in user derived virtual sequencer class) etc.).

Drawback of p_sequencer over m_sequencer:
p_sequencer creates an additional dependency, means sequence which use p_sequencer may not be executed on all the sequencers. Sequence requires resource in sequencer class which is declared as p_sequencer (i.e. pointers to other sequencers, handle of environment class, etc.)