flectra/addons/calendar/views/mail_activity_views.xml

<?xml version="1.0"?>
<flectra>

    <record id="mail_activity_type_view_form" model="ir.ui.view">
        <field name="name">mail.activity.type.form.inherit.calendar</field>
        <field name="model">mail.activity.type</field>
        <field name="inherit_id" ref="mail.mail_activity_type_view_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='category']" position="attributes">
                <attribute name="invisible">0</attribute>
            </xpath>
        </field>
    </record>

    <record id="mail_activity_view_form_popup" model="ir.ui.view">
        <field name="name">mail.activity.form.inherit.calendar</field>
        <field name="model">mail.activity</field>
        <field name="inherit_id" ref="mail.mail_activity_view_form_popup"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='date_deadline']" position="attributes">
                  <attribute name="attrs">{'invisible': [('activity_category', '=', 'meeting')]}</attribute>
            </xpath>
            <xpath expr="//field[@name='user_id']" position="attributes">
                  <attribute name="attrs">{'invisible': [('activity_category', '=', 'meeting')]}</attribute>
            </xpath>
            <xpath expr="//button[@name='action_close_dialog']" position="attributes">
                  <attribute name="attrs">{'invisible': [('activity_category', '=', 'meeting')]}</attribute>
            </xpath>
            <xpath expr="//button[@name='action_done']" position="attributes">
                  <attribute name="attrs">{'invisible': [('activity_category', '=', 'meeting')]}</attribute>
            </xpath>
            <xpath expr="//button[@special='cancel']" position="attributes">
                  <attribute name="attrs">{'invisible': [('activity_category', '=', 'meeting')]}</attribute>
            </xpath>
            <xpath expr="//button[@name='action_close_dialog']" position="before">
                <button string="Schedule a meeting"
                    attrs="{'invisible': [('activity_category', '!=', 'meeting')]}"
                    name="action_create_calendar_event"
                    type="object"
                    class="btn-primary"/>
            </xpath>
        </field>
    </record>

</flectra>