Sample package.xml:

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <!-- 
 Workflow includes WorkflowFieldUpdate,
 WorkflowAlert, WorkflowOutboundMessage, WorkflowRule
    -->
    <types>
        <members>ObjectName</members>
        <name>Workflow</name>
    </types>  
    <types>
        <members>ObjectName.WorkflowRuleName</members>
        <name>WorkflowRule</name>
    </types>
    <types>
        <members>ObjectName.FieldUpdateName</members>
        <name>WorkflowFieldUpdate</name>
    </types>
    <types>
        <members>ObjectName.EmailAlertName</members>
        <name>WorkflowAlert</name>
    </types>
    <types>
        <members>ObjectName.OutboundMessageName</members>
        <name>WorkflowOutboundMessage</name>
    </types>
    <types>
        <members>ObjectName.TaskName</members>
        <name>WorkflowTask</name>
    </types>
    <version>28.0</version>
</Package>