Enter this command to get an armored skull
/give @p skeleton_skull{AttributeModifiers:[{AttributeName:"generic.armor",Amount:3,Slot:head,Name:"generic.armor",UUID:[​I;-121924,11361,102021,-22722]},{AttributeName:"generic.movement_speed",Amount:-0.01,Slot:head,Name:"generic.movement_speed",UUID:[​I;-121924,11461,102021,-22922]},{AttributeName:"generic.knockback_resistance",Amount:0.09,Slot:head,Name:"generic.knockback_resistance",UUID:[​I;-121924,11561,102021,-23122]}],display:{Name:'[{"text":"Armoured Skull","italic":false}]'},HideFlags:2} 1

This is for the heavy armoured skull:
/give @p skeleton_skull{AttributeModifiers:[{AttributeName:"generic.armor",Amount:4,Slot:head,Name:"generic.armor",UUID:[​I;-121924,14661,102021,-29322]},{AttributeName:"generic.movement_speed",Amount:-0.07,Slot:head,Name:"generic.movement_speed",UUID:[​I;-121924,14761,102021,-29522]},{AttributeName:"generic.knockback_resistance",Amount:0.9,Slot:head,Name:"generic.knockback_resistance",UUID:[​I;-121924,14861,102021,-29722]}],display:{Name:'[{"text":"Heavy Armoured Skull","italic":false}]'},HideFlags:2} 1

Enjoy!