Difference between class alv and function alv in SAP ABAP?

Class alv and Function alv are different in terms of features. Below is the difference:
Class alv are secured as compared to function alv.
While using class alv, it improves the performance.
With use of function alv, you can create screens using function module however you need to call separate programs to generate screen.
Class alv provides Object Oriented functionality and hence they are easily reusable.
You can execute function modules asynchronously and can also be called by other systems remotely.

Below is an example of class ALV:

DATA: lcl_alv TYPE REF TO cl_gui_alv_grid,
      t_gly TYPE STANDARD TABLE OF Travels .
SELECT * FROM Travels INTO TABLE t_gly.
CREATE OBJECT lcl_alv
    EXPORTING I_PARENT = cl_gui_container=>screen0.
CALL METHOD lcl_alv->set_table_for_first_display
    EXPORTING
       I_STRUCTURE_NAME = 'Travels'
    CHANGING
       IT_OUTTAB = t_gly.
CALL SCREEN 100.

Few of function module to create ALV reports:

Sr.No	Function Module & Description
1	REUSE_ALV_LIST_DISPLAY Display an ALV list
2	REUSE_ALV_GRID_DISPLAY Display an ALV grid
3	REUSE_ALV_COMMENTARY_WRITE Output list header information
4	REUSE_ALV_VARIANT_F4 Display variant selection dialog box
5	REUSE_ALV_VARIANT_EXISTENCE Checks whether a variant exists
6	REUSE_ALV_FIELDCATALOG_MERGE Create field catalog from dictionary structure or internal table

Below shows the use of function module to display an ALV grid:

DATA: it_spfli TYPE TABLE OF spfli.
SELECT * FROM spfli INTO TABLE it_spfli.
CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'
  EXPORTING
    i_structure_name = 'SPFLI'
  TABLES
    t_outtab         = it_spfli.