Borrowing some code from Kuba's:
set = {20, 36, 70, 96, 152, 301}
Graph[DirectedEdge @@@ #,
VertexShapeFunction -> "Square",
VertexSize -> {.2, .1},
VertexLabels -> Placed["Name", Center],
VertexLabelStyle -> Directive[FontFamily -> "Arial", 10],
GraphLayout -> "CircularEmbedding",
EdgeLabels -> ((DirectedEdge[##] -> Placed[#2 - #, .85]) & @@@ #)] &@Subsets[set, {2}]

Edit
In fact, you could apply any nice formatting to the edges. The following applies a color proportional to its value:
set = {20, 36, 70, 96, 152, 301};
{min, max} = {Min@#, Max@#} &[Subtract @@@ -Subsets[set, {2}]]
Graph[Property[DirectedEdge @@ #,
EdgeStyle -> {Thick, ColorData["Rainbow"][
Rescale[#[[2]] - #[[1]], {min, max}]]}] & /@ #,
VertexShapeFunction -> "Square",
VertexSize -> {.2, .1},
VertexLabels -> Placed["Name", Center],
VertexLabelStyle -> Directive[FontFamily -> "Arial", 10],
GraphLayout -> "CircularEmbedding",
EdgeLabels -> ((DirectedEdge[##] -> Placed[#2 - #, .85]) & @@@ #)]
&@Subsets[set, {2}]
