java.sql.BatchUpdateException: Batch entry 0 insert into kv.KV_USER_ATTRIBUTES (TYPE, VALUE, NAME, USERID) values ('java.lang.String', '<stream of 11 bytes>', 'attribute1', NULL) was aborted. Call getNextException to see the cause.

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated Nov 7, 2011 11:55:23 PM by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping auto-import="false">
<class name="com.nd.core.users.User" discriminator-value="User" lazy="false">
<id name="ID" type="java.lang.Long">
<column name="ID" />
<generator class="native"></generator>
</id>
<property name="name" type="java.lang.String">
<column name="NAME" />
</property>
<property name="firstName" type="java.lang.String">
<column name="FIRSTNAME" />
</property>
<property name="lastName" type="java.lang.String">
<column name="LASTNAME" />
</property>
<property name="email" type="java.lang.String">
<column name="EMAIL" />
</property>
<property name="markedAsDeleted" type="java.lang.Boolean">
<column name="MARKED_AS_DELETED"/>
</property>
<list name="roles" inverse="true" table="KV_ROLES" lazy="true" cascade="all">
<key>
<column name="ID" />
</key>
<list-index></list-index>
<one-to-many class="com.nd.core.users.Role" />
</list>
<set name="attributes" table="KV_USER_ATTRIBUTES" lazy="true" inverse="false" cascade="all">
<key>
<column name="USERID" />
</key>
<one-to-many class="com.nd.core.users.UserAttribute" />
</set>
<union-subclass name="com.nd.core.users.impl.UserImpl" table="KV_USERS"></union-subclass>
</class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated Nov 8, 2011 12:39:38 AM by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping auto-import="false">
<class name="com.nd.core.users.UserAttribute" abstract="true" >
<composite-id>
<key-property name="name" column="NAME"></key-property>
<key-property name="userID" column="USERID"></key-property>
</composite-id>
<property name="type" type="java.lang.String">
<column name="TYPE" />
</property>
<property name="value" type="java.io.Serializable">
<column name="VALUE" />
</property>
<union-subclass name="com.nd.core.users.impl.UserAttributeImpl" table="KV_USER_ATTRIBUTES">
</union-subclass>
</class>
</hibernate-mapping>

User user = new UserImpl();
user.setFirstName("John");
user.setLastName("Doe");
user.setEmail("nirmeshd@yahoo.com");

UserAttribute userAttribute = new UserAttributeImpl();
userAttribute.setName("attribute1");
userAttribute.setValue("test");

UserAttribute userAttribute2 = new UserAttributeImpl();
userAttribute2.setName("attribute2");
userAttribute2.setValue("test2");

Set< UserAttribute> userAttributes = new HashSet<UserAttribute>();

userAttributes.add(userAttribute);
userAttributes.add(userAttribute2);

user.setAttributes(userAttributes);

Long _insertedUserID = um.insertUser(user);

assertTrue("null user id returned",_insertedUserID != null);

User userFound = um.getUser(_insertedUserID);

assertTrue("user not found after insert",userFound != null);

Might be helpful to see the whole stack trace.

Also, what is "um"? UserManager? How is it implemented?