SystemVerilog Demystified.

Virtual (Abstract) vs Concrete

The clone method is used to provide a deep (nested) copy of an object. clone first allocates new memory for the object, then copies over each field to the new object. If a field is an object handle, then instead of copying the handle (which would do a "shallow" copy) you would call fieldname.clone() to recursively allocate memory for that field (a "deep" copy).

Clone (Virtual) vs Copy (Concrete)

class base;
int p1;
  function void copy(base orig);
    this.p1 = orig.p1;
  endfunction
endclass
class ex_base;
  int p2;
  function void copy(base orig);
    super.copy(b);
    this.p2 = orig.p2;
  endfunction
endclass
 
base b1,b2;
ex_base eb1, eb2;
initial begin
   eb1 = new; eb2 = new();
   eb2.p2 = 5;
   b1 = eb1; b2 = eb2;
   b1.copy(b2); // p2 is not copied
   eb1.copy(eb2); // p2 is copied
end

// Since copy() is not virtual, calling b1.copy() calls base::copy(), 
// and the additional property p2 is not copied even though it exists 
// in object referenced by b1.